My '02 Burb's battery drains over time. I checked every circuit in the car. You put a multi-meter set to amps between the ground and battery, then remove every fuse one by one until you see the amps drop. That circuit will be the one with the problem. For older cars the amperage when the car is off should be under 20milliamps. That's to power the clock and the remote locking receiver. On newer cars under 50milliamps is considered normal. Our GMT800s are right on the cusp of being a newer vs older car. But I was unable to find which circuit was drawing the current. Maybe I needed to wait for the accessory power to time out. I may try again.
Since I only use mine for towing my race car, or hauling stuff, it sits for weeks or months without being started. Even a tiny amp draw can drain the battery to the point where the battery is damaged. So I installed a battery cutoff switch. So when I park the Burb, I pop the hood, then lock the doors with the fob, then I disconnect the battery with the switch, and close the hood. My battery has lasted a year with no issues since then. Yeah my clock always reads wrong, and the factory alarm is not functional, but I don't care about those things. What I do care about is that when I need the thing, it starts right up with just a little hassle. Obviously this would be a lousy solution for a daily driver.