BMW battery dies with passenger door use

My son’s 2008 bmw 528i .... for 6 months the car’s battery intermittently dies for no known reason.... garage had for almost 1 month testing for parasitic drains or electrical issues. While he was at school I (I’m his Mom) had to use his car.... what I noticed is when the front passenger door was opened and closed-the next day battery completely dead .... notice this happened again so went for days without using passenger door and battery never died ..... he had back at school and used passenger door once in a month and that is when battery died.
Going back to garage .... it cost us $950 the 1st try to figure out .... really would love the cause to save money on garage trying to figure out ..... THANKS IN ADVANCE - Michele