Well the cable is pointing straight down. I figured they snipped part of it and/or spun the rest until it broke. If the tire had dangled and fell off while driving I doubt the cable would be that straight. I would think it would have done some banging around before falling. Also for a cable to spin itself sounds highly unlikely.
I cannot rule out that the tire may have fallen off but it would seem highly unlikely. That is why I wanted to post and see if anyone else has had this happen. Has anyone checked the cable on yours lately?
The only reason I question it being stolen is that the little winch and cable are enclosed behind the wheel/tire when it's mounted up there, in order to get to the cable to cut it the tire would have to be lowered and at that point they could've just removed the spare off of the retainer without cutting anything, but maybe they didn't know that. Thieves aren't known to be the smartest people.
Maybe they somehow forced the wheel down to get to the cable, but I'd expect the little winch to break.
Anyway, not saying one way or the other. Who knows what someone would do to try and steal a spare. I also would've expected you to notice if something caused it to fall off while driving.
That said, you make an interesting point about it being spun until the cable broke. It's entirely possible that the tire wasn't tight against the underside and the wind was causing it to spin as you drove before it finally fell off. With how quiet/insulated these trucks are I don't know that you'd hear/feel a light bump from the tire being a little loose?
Sucks regardless and I'm just bored while waiting for others to do their work so don't mind me.

Hope you get an answer and find a cheaper fix/replacement!