Some cats will get up in the lining of the mattres and hide there.
That's where my cats hide.
Look underneath the couch and inside recliners too.
Snakes, too.
When growing up, my oldest sister kept a few snakes. Knowing my mom (now) I can't imagine her ever letting snakes in the house. But, one of the snakes disappeared. It was a good week later that it was found in the mattress (still very much alive and well). Just about gave my mom a heart attack at the time.