Seriously those monsters aren't just always standing around, or playing cards. Maybe they got a hot tip that some fresh meat is headed there way.
1. Spring loaded floor - Mucho surprise! Much like the loaded floors at theatres when the performers pop out of nowhere, here's a perfect surprise.
2. Ye old fake bookcase
3. Behind the curtains - sure the PC's might see there feet.
4. In the barrel - perfect place for a goblin to hang out in.
5. Invisible! - Sure why not, its been done, do it again.
6. Teleport - Suprise the PC's by appearing out of thin air.
7. The ceiling - totally obvious by how many times do you look up?
8. Hiding inside the pit trap. - Maybe the goblin is hanging halfway down the pit trap waiting to jump on an already wounded PC.
9. Under the bed - There's a reason little kids are afraid.
10. Disguise - maybe the monsters are dressed up to resemble something else, possibly a random merchant or a band of dwarven miners.