David I had a same issue with my FX last year. I went overseas, and while there my family started having charging issues, the car would die if sat for more than a day without being driven. I though they must have been doing something wrong. Anyways 2 months later I returned and it got to the point that even I couldn't get it started without charging the battery. The system check didn't find nothing wrong, the battery when charged and car driven was fine, alternator too, all fuses fine. Finally one day I had to replace a bulb, I noticed one of the cables was ripped, not sure how. I fixed the broken headlight cable, put new lightbulb in and for some reason the problem went away.
I talked with a friend and he said that some cars need a close circuit to run system check after you turn the car off. If there is a breach or broken "line" the system won't be able to do a full "circle" and is going back and forth until it runs itself down, since it can't properly shut down.
So I would just check for any loose cables.
Other things that might be causing the drainage, since I spent a month looking for it could be: faulty stereo/nab unit. In some cases it stays on even when the screen is off. Resolution is to have it replaced.
BCM module may be dying, it's behind the glove compartment. If so you will have to have the dealer check it, replacement requires reprogram at the dealership. Hope this helps.