BUT if I don't use it for a few days, the battery will drain down to zero. I'm thinking this is a battery safety feature Apple is using to preserve battery life (I'm a drone pilot and the expensive drone batteries do the same thing--you should not charge them unless you are about to fly because they self-discharge in about 3 days to preserve long term operation).
No, there's probably something draining your battery. I routinely take mine with me if I'm away for the weekend and if I don't use it I'll only lose a few percentage points. That's on an M1 Air. My previous Pro was the same, for the most part, though there were some minor drain issues on that one, would lose perhaps 20% over a weekend without use.