One Ring To Disgust Them All

This posting is directed at one of my colleagues. You know who you are, even if I don't.
Here's the thing. When I went to the toilet on our floor the other day, I noticed something icky. No, not that kind of icky, but still. Look at the picture:

Can you see it? Here is a closeup:

That's right. It's a ring left by a coffee mug.
A coffee mug. On the TOILET. Someone in my office is having COFFEE on the TOILET.
Now, I would be lying if I said I was the most hygienic person on the planet. I occasionally take a book into the lavatory. But this, this is taking the meaning of 'gross' to a whole new level. Does this guy (now why do I think it's a guy? I wonder) also eat his sandwiches in the loo? Is he a very loud slurper, or does he have some weird phobia about people watching him drink?

You know what, on second thought, I don't even wanna know. Note to self: BUY YOUR OWN PERSONAL COFFEE MUG.
Ew.