All vehicles have a little drain happening with the key off. The vehicle computers stay awake, even when you don't. This could show up on your voltmeter as 12 volts, but it's meaningless. You need to get an ammeter to check the current flow. I think that 40 milliamps or so is ok. That is .040 of an ampere. If you are reading a half amp, or a full amp, then you have an excessive parasitic drain.here's a short note on the your answer I just disconnected the ground wire from the battery and hook my voltmeter between that in the terminal the cable in the terminal and I'm getting full voltage to there so it does sound like a short but would anybody know what is causing it
Get a free alternator + battery test at auto parts store like Advance or Autozone. This can help confirm your own diagnosis. I know you're not fan of this device but my daughter texts me when it's less than 13.8 to 14.4 LOLDuring the pilot driving I can see in real time the voltage charge go as high as 13.8 volts then would drop 2 volts, every few minutes it would go as low as close to 11 volts under the 12v thresh hold. Then it would spike back up to the 13.8 v . Battery is coming up to 2 years Walmart brand . Alternator was swapped out at The 110k mark on the meter that was 80k or so. So this may be age creeping up on the Alernator.