Surely this cannot be the first such question, but I have not found the appropriate keywords...
This morning, I saw one of my cats (9 year old, male, fixed) hop out of the litterbox (where he did not seem to leave anything) and wipe his behind on the floor. Otherwise his behavior is perfectly normal, except that lately he sometimes makes a noise as if trying to sneeze or bring up a hairball (without result).
This is a first, though his brother has been known to poop on the floor if I neglect cleaning the box, or fill it with a strange brand of litter. The box was clean, though the litter level was lower than average (I used up a box yesterday).
So, well, what are the likely causes? Is he likely to do it again tomorrow?
(UPDATE: a month gone by, he has not done it again.)