Abby and Hanna do that all the time - really scares me. I know they hide up inside my boxspring, the liner underneath is flimsy and even though I have stapled the thing back on, they manage to rip it down so they can sneak in.

For Hanna, I trained her to come when I ring a little bell. It is a bell off one of her old fetching toys and she loves that sound. So when I can't find her, I get the bell and ring it until she comes running. I don't know if this will work forever, but for now she comes when I ring.