Could be a few things.

Sounds like there is an appliance in the car still running ( have you tapped into the 12v source anywhere?).

The door jam sensors as suggested could also be a problem, try cleaning them anyway, it can't hurt.

Or, the battery is being charged and left to discharge for a long time, then charged again, reducing the life span.

What kind of battery is it?

I hope I helped some

Bill