I just had a thought. (Really!) Something I've run across before a couple of times is, how clean is the battery? When they get some age on them, and the out gassing starts, it settles on the case, I've seen a trickle drain across the battery between the posts. I was a volunteer police officer for 10 years and part of the time I was the motor officer. Our unit cars would sit for long periods of time between fire calls or concerts. I started cleaning the battery tops evey couple of months and flat batteries became rare.