Yes,
@BY Bob suggested that and I have set up a camera. I think it very unlikely there are cats, and they aren't 'marking' they just have relocated their toilet, but it is definitely worth checking so I have deployed a camera.
And yes! I am a Jackson Galaxy fan. It was watching him that made me think maybe there is something about the other litter boxes that caused them to be less attractive. Something scary? Not clean enough? They are pretty clean but I will do a complete deep clean once the storm passes.
Very annoying!