2014 Ford Fusion SE

I have a leased 2014 Ford Fusion. This past winter it wasn't very cold
where I lived. I keep it in a garage. When I went out one day, it would start
and the car had to be jump started. I had just driven the vehicle a day or
two prior, so it hadn't been sitting for awhile. The dealer looked at it and
couldn't explain it. Then it again two weeks ago in our hot summer. I keep it
in a garage and had driven it the two days before. When I got in, the
battery was dead and had to be jump started. The technician said that
there is a problem with the Fusions where something drains the batterys
and that the dealer should know what to do to fix the problem. I took it to
the dealer and they knew nothing about and said there wasn't a recall.
They kept it for a week and claimed they would start it, check the battery,
and run a diagnostic test to see if anything was draining the battery, but
found nothing. Obviously, the battery wouldn't just be dead for no reason. I
had gas in the car, it wasn't a cold winter, this was a recent hot summer,
there was only a one day lag in not being driven, and I did NOT leave the
lights on. Luckily, this is a leased car. I was thinking of buying it, but with
this issue I don't think so unless anyone knows what could be causing this
and get it fixed while it's still under warranty.