Surprisingly, I am aware of this. The exact same situation happened to my friend. Twice! His dog is housebroken, but he must've had an upset stomach while his owner was away. Best part is, when the dog wants to relieve himself while the owner's away, he usually does it in a corner of the room, one that would take the roomba some effort to get to. And by golly, got to it it did. On two separate occasions. And then it got to all the rooms and all the carpets.

As you implied, it's not as uncommon as one would think. Roombas need a poop detector.