There is only one secret achievement / trophy in Aliens: Colonial Marines called Easter Egg, where you just have to find some alien eggs that are painted like easter eggs. Creepy right?
You’ll find these alien easter eggs in the second level, “Battle for Sulaco”.
After you open the door with the Loader, you’ll come to an area where you have to fight some Facehuggers. Kill all of them and look for a glowing blue switch. Flip the switch and go back to the previous room, and you’ll find the hidden door that the switch opened.
Inside of this room is where you’ll find the creepy alien easter eggs, and just entering the room gets you the Easter Egg achievement / trophy.