Right next to the door to the freezer room in the basement is a shelf with four photos and an elephant toy.
There is a chance that a small alien toy will appear between two of the photos. To unlock this achievement, you need to interact with the toy before it has a chance to disappear.
Apparently you can reload the game to make the alien appear, but this didn't work for me. I was able to spawn it by shooting myself with the revolver found behind the painted wall in the basement (knock the wall down with the sledgehammer). I had to try this a few times before it finally appeared.
Credit to Senpai_Ryujin on Steam for posting a link to this photo of the alien: