I think Dan has the right idea for finding a "small" power drain. I use a test lamp (looks like an icepick) in series with the disconnected battery lug and negative post. If you have drain the lamp will light. The brighter the more drain. I then start pulling fuses until the light goes out. This will get you to the ballpark of the problem. You might also consider pulling the big wire from the alternator, just watch the light. BTW, I've always been told that a battery sitting on metal or concrete would discharge it. I've read recently that's a wives tale. Who knows???

Six volt guy living in a twelve volt world