I guess the seats gone, because it's just gone. Not sure how and what direction you'd need to go to solve that mystery ?? It would be a case of who done it.
In pic #5 you can see the sweat marks from someone's bare legs on the front edge of the seat. Wild guess...? I suspect it was used as a make-shift (that's shiFt) toilet for a person with a bowel disorder. The seat is gone so a dish pan or similar tub could be placed there for... well... you know...