Overview
In this Guide, we have almost every secret from the Half-Life 2 series. I will also show you how to find them. Enjoy!*UPDATED*
Intro:
Prologue:
In Half Life 2 there are series of secrets or hidden Easter Eggs, but first, let’s start with the mystery of the Lambda.
And what’s funny is Valve wanted to use it for Half Life, as the “a.” The Lambda’s capital form is also used in Stargate Atlantis as the middle “A” but with a circle above (represents Earth). It’s also in the name of the Disney movie Atlantis: The Lost Empire as both “A’s” Seems like the Lambda’s very well used, as it’s also in Star Wars as a name.
Half-Life 2 Secrets.
Chapter 1, Point Insertion:
First up, we have the billboard: 
Next, we have the whole huge area, if you turns cheats on and noclip out you’ll see the huge area for yourself, but the main part here is a mini-town.
Chapter 4: Water Hazard:
This chapter may have more than one secret in-cased
Recently, I found this melon, well, someone found it, and YouTubed it, and I went to find it in HL2, and here it is! If you go to the part were you face a hunter chopper, the area with Metros and a loft area that has a button to open the further on gate, and two mounted guns, it’s in that complex, somewhere in a dumpster kinda near where you park your airboat, it’s possible to reach without cheats.
The main one that pretty much everyone knows of is the Hidden Singing Vortigaunt, finding him will gain an achievement, “Vorticough”
Chapter 5: Black Mesa East:
At the Beginning of the chapter, once you go down to meet Eli for the first time since HL1, you’ll notice a strange thing in a glass jar, it’s actually a cut enemy from Half Life 2, but that is just it’s head, even if it was cut, this would still imply that the cut enemy, the Cremator, would be in the Half Life universe, even if we’d never find it apart from it’s head in a jar. (I’ve got a jar of dirt)
As for now, these are the only secrets I can come up with, but just to think, there are so many out there in the Half Life universe, no one truly can pin-point every last one except for the Developers themselves. -End of Section
Half-Life 2: Episode 1 Secrets.
In Half Life 2 Episode 1, there are only a few easter eggs in Ep1.
Chapter 1: Undue Alarm:
Basically, after D0G throws the van, and you climb in, look at the steering wheel, there’s a picture of a bear, it’s a Radiohead reference, not certain if it’s official.
Chapter 3: Low Life:
Excuse the fact that I had mods on, I almost completely forgot about it until I realized after I took the picture. Ehem, this easter egg is rather interesting, and I also forgot to add it in, heh my laziness made sure of that, it’s pretty much a bar code, take a glance:
Chapter 4: Urban Flight:
In the Hospital area of Episode 1, on the walls, you notice that a sign next to small open patient rooms are the names of the game’s developers.
Chapter 5: Exit C17:
Near the ending, after you lead the citizens into the safe room, and obviously that same area you get a loading screen, before you fight the strider, there’s graffiti which is actually an equation for finding the Resistance of a material, sorry no picture, it’s also in Ep2.
And there we have it, the only thing I could find for Episode 1. -End of Section
Half-Life 2: Episode 2 Secrets
Half Life 2 Episode 2: Here it finally get’s interesting, Ep2 is simply bursting with Easter Eggs! Well, that’s just a figure of speech, it still has plenty that I can name like the back of my hand more than HL2.
Chapter 1: To the White Forest:
The gnome, we all know this little monster, (the pain it is taking it throughout the game) It is hidden under the storage thingy in the transmission room shortly after fighting some zombies in the previous area. This gnome possesses a special power, when you get to the car, but rather go to chapter Riding Shotgun, you’ll find that if you jump on the gnome, it’ll cause you to fly up in the air, when you fall down, you’ll either die, or the 0.5% chance of survival.
In this same chapter after leaving the Vortigaunt with Alyx, your in the area which you crank your way up the small shaft, I recently found this by accident, when cranking, crank until you absolutly reach the top, you’ll go past this little cache, but when you drop back down, you should be able to grab the contents inside, via gravity gun.
Chapter 2: This Vortical Coil:
In this chapter we have a little easter egg hidden down the track, it’s rather hard to come by, as you may need to cheat to get to it, it’s somewhere along the tunnels when being chased by the acid guardian, you’ll need to go back to this certain area after you go through the entire tunnels, just use noclip to return to the start and find this area with stalicmites blocking it off. It reads: “How did you get back here?!”
Chapter 6: Our Mutual Fiend:
This is the chapter with the major easter eggs. At the beginning, you’ll find these guys discussion the AR3, the AR3 doesn’t even exist, but I’d give my suspicions to Half Life 3.
We have the Lost easter egg later on in this same chapter, found in the room with Uriah just after leaving the first silo behind the small tanks. The computer seems to have numbers on it which refer to the numbers on the computer in Lost. There is on the wall a symbol with the White Forest logo inside of it, this symbol by iteself though is also in Lost. If you go to the Developer Commentary you’ll get to see a commentary node inside this room, in which you can hear what the developers have to say about it.
Later within this same chapter can be found a room and collosial area, go to Judith’s transmission area where Alyx has her little message to Eli and noclip from there outwards, you should see these two as clear as day, go to them and view it for yourself.
In this same chapter and main location (transmission room) while the Borealis blueprints are fowarding you can see this: 
Around in the littler storage area with crates and stuff you can blow it up with a granade and find the HEV suit charger faceplate again like we did in Half Life 2 before Ravenholm.
-End of Section
The Alyx Affection
Alright, so pretty much, let me give it to you completely straight forwards as calmly as I can, this is a secret, not really an easter egg, it’s literally RARE (don’t know how many people have actually come across it). Don’t get too excited, you may be shocked actually:
Within Episode 1 and Episode 2, Alyx does a little flirt with Gordon. . . . . . O.O well….isn’t that just splended! Basically, if you’re (can I say just Gordon?), if Gordon is standing behind Alyx, when she’s doing something, and what not, then she turns around, to face him (though he’d technically be standing pretty close), she decides to do this:
Alright, I just find that rather creepy, can I go kill her now? (I mean, not that I hate Alyx or anything, just that because I’m a gal myself, I feel obliged to punch her nose, lol) This is something I totally found on my own, I DON”T KNOW HOW? I must’ve been standing too close to her when she turned about and did that, which case my reaction was a total freak out, just for once, can I be someone who isn’t a guy?
Here’s a little GIF of her doing it in action: [link]
It’s something we have never seen Alyx do before, it’s quite surprising to me, well, goes to show her affection towards Gordon. 😛
The Consoling Couple:
Half-Life 2:
This here is a man consoling his wife over the Combine, something like that, and it appears that this couple makes their way throughout the Half Life series, if you’ve ever seen two people sitting on a couch future onwards, that them.
In Route Canal, there appears to be the same couple being threatened by the Metrocops, the man is beaten down when the player arrives, these are the same models used back in the apartments, it could be that these are the same people, but the man obviously didn’t die if they were.
Later on, in chapter Anticitizen One, you can find this same looking couple on a couch again, this time, they are wearing Resistance uniforms, and the man has a beard.
Half-Life 2: Episode 1:
In episode 1, were you’re in the TV room (Urban Flight), you’ll evidently notice this very same couple, at first you’ll just see them standing up, but they too have the same models as previously, once you go upstairs, and come back down, they are there sitting on the couch. Later on up, if you fall down into the zombie pit, go through the door, you’ll notice a small room which you can spy through on the couple downstairs, they’ll talk about Odessa Cubbage, the Combine, and Dr. Kleiner.
Half-Life 2: Episode 2:
First off, in chapter Under the Radar, you get the car fixed and drive onwards to the first cache, but before that area, you should’ve noticed a small area where some zombine comes out of, when driving, it’ll be on the right, go inside the building and you’ll see this which very much resembles that same couple, but as skeletons, they are even in the similar pose. (To me, it looks like one of them is biting the other, and the other is screaming) but it turns out that this isn’t the same people, as they make appearance at White Forest Base.
In White Forest, chapter Our Mutual Fiend, of course at the very beginning, just before meeting Eli, in this room you’ll see the same couple again! So they evidently survived everything, and showed up at White Forest in the end. If they some how appear in Half Life 3, I’d have the feeling that they are spying on us. 
The Videos:
Here’s the video section, My two videos have been deleted by the user of that channel I had them on, sorry, maybe in the near future I will have those videos back, but on a different channel. Stay tuned!
Outro:
Epilogue:
Well I guess that’s everything I could dig up, there are so many little hidden things which only the watchful eye can spy. “The truth is out there!” Don’t forget to give it a rate! 🙂
-Ekstiraptor