Remind of which species you have again? I've heard chineseys are particularly prone to using their wheels as a toilet so if you have a Chinese, it wouldn't be all that unusual. It's also not unusual in the other species though so I wouldn't be surprised either. Chances are, if after looking all over, you still can't find it. Then I wouldn't stress over it. Just wipe the wheel down with a wet cloth once a week or so
