Dead Battery

I have a 96 Ford Explorer I recently had the transmission rebuilt and when
I purchased the truck to motor had been rebuilt. When I bought the car
the dealer had to jump the car to start because the battery was dead, I
thought nothing of it but when I had it at the shop getting work done and
went to pick it up I had to jump it. I just purchased a new battery and
put it in about 3 weeks ago, my car has been sitting in my parking because
my breaks need work and when I went to go start it today there was nothing
battery dead. Is this because it is so cold outside and my truck is just
sitting there, or can I have a short somewhere? It's only when the truck
is left sitting for a few days that this happens. Any thoughts would be
great. Thank You..