the mark of the chimney sweep

For the last few days I've noticed about mid afternoon that I've aquired a black smudge on my wrist above my thumb. I imagine that it looks like the sort of smudge you get if you are a chimney sweep but since my chimney sweep smudge experience consists entirely of watching "Marry Poppins" I could be wrong about their similarity. Anyway, I have no idea where it comes from, it's not as if I have a chimney that I may have been inadvertently sweeping when I wasn't paying attention. I'm actually in London at the moment but I've been madly working on my new website (a link to which I'll be publishing here soon) so my daily routine has been something like: get up, watch tv/work on website, eat bagel, work on website, and then mid afternoon I notice the black smudge.
As you can see there is no activity that seems obvious for black smudge acquisition. I did do a bit of laundry one day but since my laundry is smudge free it seems unlikely to have come from that. Today I seem to be smudge free (thus far at least) and Kim made toast instead of me but he is smudge free so I don't see how the toaster could be the culprit.
It's a mystery. But as long as the black smudges don't start covering increasingly large sections of my body or become immune to soap or cause me to have strange powers like the ability to levitate I suppose it isn't to big of a problem.