In another thread there's confirmation about the locked trucks in Nashville.
https://forums.macrumors.com/showthread.php?p=14500772&+fedex+freight+#post14500772
My dad works for FedEx Freight and drives to Nashville every night. He says that they're in trucks that are by themselves and locked. Not many of the drivers can open the trailers, or at least my dad doesn't know anyone who knows anyone who can open them.
I stand corrected. That's some interesting info. Very cool.