Did he have a plastic carrier bag over the saddle?
We had one guy at work who used to tend to his horse before he came to work. He would then go into the disbaled toilets and knock all the mud off his boots leaving it everywhere.
Guess who got the blame? Well it must be the bloke on the mountain bike mustn’t it. So despite me cleaning up after myself rigorously after getting a strip wash in the disabled toilets (and I was never muddy), I got banned from getting a strip wash in there.
Strangely this didn’t stop the mud getting in there and it was only after he left and the mud strangely stopped, I was allowed to start commuting again.