I can top the OP's story.
Years ago I was flying EZE-MIA on 777 in 1/2 empty J cabin.
I was in 13D (13E and 13F wree completely empty at EZE when we took off).
Anyway, around 1/2 way (maybe 3/4) thru the flight, I went to use restroom.
When I came back, someone was magically sitting in 13F. Not only that, 1/2 my pillow stash was gone.
I went to the J Galley, and talked with the very nice FA who had earlier served me dinner. The FA said he never saw anyone in 13F (or 13E) while he served the dinner, and double-checked the pax list with seat assignments. He also told me no one was assigned to either seat. Furthermore, he explained that international J and F FAs are trained to memorize the cabin (i.e. where are people sitting).
He immediately went back to 13F, and wasted no time in waking up the Y pax who was camped out (with all my pillows) in 13F. The FA immediately kicked him out.
Yes - the Y pax had the nerve to take the J class pillows with him back to Y:
. He forgot his blanket though...