I have had occasional problems with battery draining completely overnight for months, a trickle charger helpsmthings.
Recently I have had the self starting syndrome, where the starter would not stop spinning, even without key in the ignition. Came home Sunday night, after driving all day in the rain, parked in the garage, shut her down, and starter was engaged. I had to unhookmbattery to make it stop.
Towed to my mechanic, he thought starter for sure, solenoid is stuck. Ok, sounds good. But my starter is reman Bosch, 14 months old.
Tech started looking at cables, and the cable from starter to alternator was toast. He spilt it open, and you can see burnt ends and they were visibly toching causing the issue.
Some new cables, and cleaning up, the car is fine. No need for a starter, just 2 hours labor.
Many threads here aboutnthis, CHECK your cables, I had him check all of them, this was the bad one.
Happy motoring.
Hopefulyit cures my mysterious battery drain.