Like the description stated, there are a couple of easters to find. One of them is easy, the others might be a bit more tricky to find.

I have found the first one, the easy one I guess.

The exploding λ logo
We all know this symbol when playing the Half-Life 2 campaign, right, This symbol also appears in some of the workshop maps by people when adding hidden things such as secret passages or stuff like that. There is this symbol behind the water platform, on of its corners.
This time, no passages, platforms or loot. You can interact with it, it'll play a "Nooo!" voiceline and explodes a bunch of times (You can die if you remain too close for too long).

I'll update this post if the others are found. I've tried to shoot at some of the fans, nothing happens. I've tried to noclip through each fans, nothing, I've quick-scanned each of the floating things at the limit of the map on the water below, thanks to an addon made by a pal, nothing neither. Not sure if there's anything with the mirror, as it cannot be deleted (But hey, it's a mirror, it's cool when there's one)

If you guys find the others, don't hesitate to share your findings below!