They are a self locking nut, and my big hands cannot spin them loose...need a wrench myself.
They are not supposed to be reused, one time, then replaced.
I can't say for sure, but if I was placing a bet on it, someone tried to steal it. Parts are expensive, and thieves will steal anything and everything.
A friend went out to go to work a few years ago, his pickup was in the driveway, minus the box, tailgate, rear bumper, rear wheels, brakes, diff, driveshaft, gas tank, and a few more things. Rear frame rails on the ground, he told methat he stared at the remaining truck for several minutes in disbelief, before even calling 911. Cops showed up hours later, and told him that major parts are stolen off vehicles regularly, even off cop cars while parked. I was shopping one time, came out to find the gas tank, seat, rear fender and lights, bars, and headlight missing from my DR650, parked in a busy parklot, mid day.
Parked my truck one day at the veterinarian parking lot for less than 5 minutes, came out and the tailgate was gone.