Voltage leak?

Hello there, put new battery in car about month ago, old one was, well old. All was well went to Mopars in the Park here (Farmington, MN) both days. Dart then sat for about 3 weeks, went to start and wouldn't even turn over. Charged over night next day was fine. Had new battery tested was told it's ok but not great battery, not much in reserve he said for sitting for periods of time. (It was cheap, my bad) I've been testing battery morning and nite, if I leaved hooked up dropping from 13 to 12.8 then later that day to 12.6. If I take off neg, I don't lose any amps and actually gain some back over nite. Could it be battery or do I have a elec draw when car is off some where? And if so what's best way to test for elec draw? I can't believe there's a draw there's nothing on it, starter switch maybe? I'll check back later, thanks Mark