I guess it would be the accessory belt. The location you describe is where the belt is, I've just heard it called a serpentine belt whenever anyone's talking about it.
We never had an issue with the belt coming off until the June 2010 a/c compressor trouble/repair. When the a/c clutch seized up, the belt snapped. I can't remember the exact timing, but it seems like it was about two months after the a/c work that the belt first slipped. I know there are quite a few pulleys that this belt goes around...including the a/c pulley. Is it possible that something could be not quite right with the a/c pulley allowing the belt to slip when wet? The a/c system seems to work fine, everything turns freely.
I'm not entirely sure how long we plan to keep this car, but while we're car shopping it would be nice if I didn't have to worry about this belt popping off every time it rains. I just don't want to have my husband replace the tensioner pulley and belt if the a/c pulley is more likely to be the problem. Of course, I suppose we could have just bought a wrong size belt...I didn't realize until this morning that Auto Zone sells quite a few "serpentine belts" that are supposed to fit this car, and they're all different lengths. Ranging from 25" to 88". Advance Auto only shows two serpentine belts for the ZX2, one 85.5" and one 85.433".
Maybe just replace the belt, spray the car down with water and see if the belt slips. If it does, replace the tensioner pulley and spray again? The spray test should answer the question for us. Even with the plastic thing on in the wheel well the belt slips off with water from washing the car.