Dampness on Drywall around Shower

I recently noticed dampness in the wall around my shower and don't have a clue what it might be. It's just shown up so I don't think it's just water splashing onto the walls and also the fact that it is taking quite a long time to dry. i live in an apartment with another apartment on top of me. could there be a leak somewhere?

Here's the link to the pictures I took:

http://pg.photos.yahoo.com/ph/cukababy/album?.dir=/32b2

Please help!

I would think it would be above somewhere.

With stacking showers, I've found:
Leaking p-trap,
Broken shower head arm,
Valve plate needing caulk
Shower door needing caulk
Leaking angle stop
And a few others.
Good luck on your search.