That sounds like the wire that Viperwolf described. It's slightly larger than most on the truck, should be Black with a red stripe (if you look close enough that is) and has a larger-than-normal ring terminal molded into it's end.
If that molded "block" is there at the ring terminal, that's your factory fusible link.
So far your description sounds consistent with factory wiring on Ford vehicles with ammeters instead of charge-indicator lights on the dash.
As mentioned, it's a loop and the other end is the Black w/yellow charge wire from the back of the alternator. All the power in the truck is pulled from that wire loop at various places with one or two splices.
You can check the backside of your ignition switch too, to make sure that it's not staying partially connected when in the "OFF" position. I doubt it, but it's possible.
Also pull fuses out of your fuse panel one at a time until they're all out, to see if any of them stops the drain.
As mentioned, this kind of drain ends up being the alternator or regulator around here (at least 80% of the time it seems like), so yeah, it's a common thing. But when you pulled the wires off the back of the alternator and also off the regulator and the drain was still there, that kind of ruled out the simple fix and just means you get to stay busy looking for PO booby-traps!
Gotta love those.
Paul