If this ever happens to you I want to make sure you know what to do!There are several ways to catch a loose hammie.
What room could my hamster be in?
First don't panic!Take your hamster's favorite treats(Sunflower seeds work good)Put six in a little pile,in each room.check every 20 minutes on the piles.If some are missing you know you're hamster is in that room.
Now how do I catch him?
~Take a bucket not too deep so if they get in there they can't get out.Put books leading to the top of the bucket.(Like starirs)And put they're favorite food in the bucket.When they get hungry they will smell the food and go up the stairs and jumps into the pail(hopefully)Then your trapped work
Also you could use a piece of wood as a rampuo to the bucket instead of books.
~Put food leading to the cage and put the cage in the middle of the floor of the room they are in.Every 20 minutes or so,see if they wondered into their cage.
~Put their cage on the floor with the door open(this won't work with fish tanks)when they get hungry or tired they might go into their cage.