When a kid is still wearing diapers and not potty trained and they go and hide when they need to poop in the diaper it usually means that the child is aware of what's going on and they need some privacy to poop.
My kid when he was 3 would start hiding behind the chair, couch or go under the dining room table to poop in his diaper and he did so because he just wanted privacy.
Then I would ask him if he pooped his diaper and he would tell me no but he did poop the diaper and need changed.
He still wasn't quite ready to use the toilet him hiding when pooping the diaper meant he was near ready.
Some kids may be ready to potty train when they hide and poop in their diaper so it's worth a try to get them to use the toilet.