Power issue on a BMW 528xi

Car doors were left open while at the car
wash and the battery went dead. They
jump started the car; I went to the
mechanic he tested the battery and it
was going up to normal range. I was able
to start my car the following morning
however, when I arrived to work I tried to
start my caravan while parked; It would
not start. I had a dead battery. Any ideas
as to what could be wrong? No battery
light on the dash however because I had
no power every indicator came on.
Mechanic suggested to change the
battery if it continued to happen