We Were Here

You can find the Easter Egg in the Dungeon Puzzle as the Explorer.
You have to give the switch a "Happy Ending"...

Interact with the Pink Switch multiple times. And then some confetti will pop out and you will unlock your Achievement🏆
(Only the Explorer gets the Achievement)
Here is my Video Guide:

‪This is a 2 player online only co-op game.
One player plays as the explorer and the other is the librarian
You need to do 2 and a half playthroughs to get all the achievements
The game takes about 15-20 mins to finish a playthrough
You have to playthrough once as the explorer then once as the librarian
Then you need to play up to the dungeon for the last achievement
You need to do each puzzle perfect the first time for an achievement
You also need to do a play through without dying
My video is 18 mins long and it will guide both sides through simultaneously
The explorer does most of the work

For this achievement you need to reach the dungeon and flip the red switch about 10 times until confetti pops out. Do this on your third playthrough. See in video at 17:15
*Only the explorer gets the achievement*

Achievement Walkthrough: ‬