I found this easter egg
The invisibility++ plasmid exist in the game, but you need to do some crazy stuff to get it. It will give you instant invisibility the moment you stop moving (normal invisibility will only take effect about after 1~2 seconds), and it will only lose ability after 5 seconds of movement! for a good slow player, he can take advantage and be invisible the whole level

To get this, you must go to Sander Cohen's level. He will give you a task of taking pictures of 3 person. You will take picture of a pianist at first. after this he will send you around the level to search for the others. Now, just find any 2 ppl, kill them and take their pictures. DO NOT PASTE THE PICTURE AT THE STAGE IN BETWEEN! This mean, kill one person, take picture, kill another take picture and THEN paste it. Cohen will then send some splicers at you. Kill them. Now go find cohen's dressing room/movie room. It's on the topmost floor. You cannot enter it because he will say he's not ready yet. now look around the room you are in now. There will be a guitar near the door. This is the most tricky part. According to your cursor, the guitar will make different sound. Go to the internet, and search for "Twinkle Twinkle little star music notes (the ones where it tells you which cord to play)". Using this, TRY TO PLAY THE SONG ON THE GUITAR. If you're successfull, you will get the plasmid.
Have fun