What is this?

I recently picked up a cheap 7040 SFF PC to build as an office rig. With 32GB RAM still in the mail I decided to pull the thing apart and give it a deep clean in preparation of putting it all back together.

On removal of the power supply, I found this little rubber thingy just chillin. I cannot for the life of me figure out where it would live inside the case… or is it an intruder from beyond?