2007 Ford Mustang My car battery Dies for no reason, Help!

2007 Ford Mustang

I have an 07 Mustang that I bought in the winter, got it home and I was off for 2 days and the battery died. I charged it and it happen again so I bought a new battery with higher Cold crank, Lasted a week then died every 2 days again. Took it to ford had to Draw tested and alt Tested and nothing, didnt find a thing, 2 days later died again, took it back tested again nothing, Now its still dying every two days and no one can find the problem, someone plz help.