I know when I have brought new kitties home, they are inclined to bury themselves in a tight spot and hide when anyone is home, waiting to come out until it is safe. I've found them in the weirdest spots.
If you haven't already checked (although it wouldn't hurt to recheck), you might try looking:
-INSIDE your furniture (I've pulled animals out of the back of a recliner before)
-inside kitchen cabinets
-behind the fridge
-inside closed drawers
Depending on how clean you keep your house and how often you dust, you may also be able to find a dusted trail or a spot where a cobweb is broken that will give you a clue.
Good luck man!