When my hamster disappeared, I could hear her scratching behind the wall but I couldn't get to her. This trick worked like a dream:
Screw up some sheets of newspaper then straighten them out so they are all crinkly and rustly, then sprinkle dry hamster food all over them. At bedtime, sleep on the floor of the room where the hamster disappeared, and place the newspaper sheets on the floor around your head.
Hopefully, when she comes out to feed, the sound of her walking on the crinkly paper will wake you and you can put her back in her cage and get to bed. Sounds a bit extreme but it really works and it won't scare the hamster away like some other methods.