Dead battery. There seems to be a battery draw because the battery will completely discharge after sitting for a few days.
Get your battery tested …for free at Advance Auto and others. A dead cell can show this behavior.
The dealership installed a brand new Motorcraft battery.
If you know how to use an ammeter, connect it at the battery and remove fuse by fuse to see what circuit has the problem. (It still might be elsewhere.)
How long ago? Batteries can and do fail. Just trying to cover all the bases.
The new battery was installed on June 23 of this year.
Okay, it’s unlikely a 2 month old battery would fail already. You probably need to have the vehicle diagnosed for a “parasitic draw,” where there’s something is draining the battery even when the vehicle isn’t operating.