[68C15]

It's possible the ignition switch is causing a draw after a while being unused. I have trouble explaining it but I can see it in my mind.

Put ammeter between cable and battery then check it every couple hours. If/when you see it above 20mA wiggle key to exercise the sliding contact in switch and see if draw goes away.

I check parasitic draw by using a digital meter and check voltage drop at fuses. I have a chart at work showing acceptable readings for each fuse rating.