Might be a long shot, but if it was converted to top post or if you have a battery with provisions for both top and side, I have seen a surprising amount of parasitic drain across the top of the battery. It happens frequently and is often over-looked. Either someone will clean the battery and it leaves residue (almost 100% of the time when someone cleans it with Coke), the battery will start leaking out of the posts, or it just gets some kind of conductive crap on the top and will bridge the terminals. A really bad case can certainly drain a battery over night, and it's very easy to check for. Just take a DMM and place one lead on the positive post and the other in various places across the top of the battery. I can't remember specifically what our given specs were, but I'd imagine if you're registering more than a tenth or so of a volt it couldn't hurt to give it the old baking soda and warm water treatment. I've seen as much as four volts! :shock:
As far as checking for amperage draw on parasitic drain, I try to enlist the help of a friendly guy over at the NTB to use his VAT machine. It only takes a second to clip the inductive clamp on it and you sure won't harm the machine. Might be worth even slipping the guy a few bucks on the side to give a shot.
-Tony