Intermittent 12v battery problem

I know the 12V battery problem has been beaten to death through dozens of threads here but I can't find anything that addresses what I'm dealing with here. After all the typical issues I replaced the one in my 2010 with solar roof in late 2019. About 2 months ago it died on me after being parked for several days. Since then I've found it's fine if I drive it every day but if it sits longer than 2 or 3 days I have to charge it as the voltage will drop to 7 or 8VDC. Note when the problem started it was still basically winter here on the CA north coast, cold but not necessarily freezing, with lots of fog and rain and little sunshine.

Well I just got back from a 6 day trip assuming I would have to put the charger on it but much to my surprise I was able to just walk up to the back hatch and pop it open without crawling inside, which I've done countless times because the car inexplicably locks itself up every time the battery dies and I can't unlock the hatch from the outside. So before hooking up the charger I checked it and what do you know, 13V. It hasn't held a charge like that for months.

I figured all along I should have gotten at least another year out of that battery. I've load tested it with my cheap Harbor Freight toaster and it's marginal, not great but not completely kaput. I have not yet done a powered down current drain test but I guess that's next.

I'm just scratching my head about what could have been draining the battery the last couple months that all of sudden is not draining it now. Is it just because the weather has warmed up? I had gotten to wondering if perhaps there is a failed diode in the solar circuit causing it to drain at night but then I read that the solar panel isn't even connected to the battery to begin with, though I don't know if that's true or not.

Either way I'd sure rather not replace the battery now but I also don't want to get stranded somewhere because of it. The car has not been modified in any way, just stock premium stereo system with nav screen, no additional electrical loads.