Yes I had something similar in a previous job, I was 99% sure that the guy who complained about it every morning was actually the culprit!
We checked it clean one night, no one else entered as we had CCTV in the outside corridor, the next morning same guy came crying that it was disgusting and he shouldn't have to use facilities in that condition. A review of corridor CCTV showed him being the first to enter the locker room in the morning.
The weirdo left soon after.