11 year old boy soiling on bathroom mat

A couple of weeks ago, our 11 year old boy 'pooed' on the bath mat in
the middle of the night. The bath mat was folded over and put back on
the edge of the bath. I put it down to a stressful situation at home
recently and probable/possible sleepwalking. He didn't admit to it
but I spoke to him about what happened (without blaming him or
chastising) and it didn't happen again until last evening about
7.00pm. He had just been told not to spin around in the kitchen near
hot pans while dinner was cooking. No one shouted or told him off,
just made him aware what harm it could do! An hour or so later, I
went into the bathroom and found a 'poo'on the toilet mat (again
folded over) on the floor. This time I asked him about it (gently)
and he got extremely upset and locked himself in his room. He
wouldn't admit to having done this at all, although there is no other
rational explanation, and despite my talking and asking him who or
what else could have done this, he would not agree with the logic.

He's never had any such problems before and I'm at a loss! I've never
heard of this before and wondered if anyone had any views or comments.