Hi to everyone,
There are lots of useful comments in this forum. I thought I'd add some of my thoughts ( as an old aircraft maintainer ).
The battery may read 12 volts even when it is weak. Voltage is not the issue here. Current is the issue. When the car is off everything should be OFF with exception of a few minor current drains for the clock, etc. The designer takes this into account. If you have a short, it will drain the battery very quickly...usually a bare wire somewhere, touching the frame ( aka GROUND ) and you may have sparks and...well!
Little things like sticking relays manifest themselves in funny ways.
If you were closer to Seattle I'd pop over and help you troubleshoot the problem.
Seems that your local dealer either doesn't care about service or his mechanics need some training. You would believe that after the new Bird has been around for four model years that he has one guy who knows the car.
Sounds like a sticky relay is the culprit, allowing a load (lamp, heating element, etc.) to drain the battery. The bigger the load, the faster the drain. The dealer should have no trouble testing current drain and probably has a spec on it.
When storing a vehicle ( more than one month ) the battery should be disconnected or you should use a
trickle charger, or George's suggestion of the
battery tender.
Hope this helps. Take a big stick with you the next time you visit the dealer!!
Putt
😎 😎