They've had these problems before at both the Engine and Transmission plants.
It was an outside contractor. They barred all contractors from using plant facilities, problem didn't stop. Then they set up cameras and bathroom proctors. Problem solved and that contractor was barred from all plants.
Reminds me of the mid 2000's working at a certain 3 letter agency and the poo bandit. They would make smiley faces in the stalls with their fecal matter. I always wondered what that turd burglar looks like and did he wear gloves? So many random questions unanswered........but was always the talk of the town/agency when he struck