It's fairly common. Wouldn't you be mortified if you found you'd "gone" in your pants - and try and clean it up yourself, rather than go and ask someone else to come and clean it up?! Unfortunately, with dementia steering things, "normal" ways of cleaning up don't come into it. The problem - making the mess go away. The solution is to just put it somewhere it won't be seen or found. Problem solved, as far as dementia is concerned.
My husband used to do the same thing. And one time I met him coming out the back door with the, er, contents of his pad in his bare hand! "I didn't know what I should do with this." he said - and handed it to me!! Another time, after a couple of weeks, I finally tracked the awful smell in our bedroom down. He had hidden "it" by using it as a sort of top dressing on the row of potted plants on the window sill, pressing it well down into the pots!! Pots and all went into the bin!
This problem is so distressing for the person with Dementia and the Carer; I'm afraid that I never found a resolution to my Husband's inventiveness in getting rid of the contents of his pad. Poo left in shoes, the garage and down the bathroom basin were all the norm to Pete. When he was extra distressed the contents were smeared over the bathroom walls, walked through carpets or smeared in the dogs ears.
I could only try to put myself in his position and I would have hated to have my personal care done by anyone. Pete was a private and proud person and I'm sure that however calm I was about the situation HE knew deep down that he had done (in his eyes) something wrong and he was trying to hide the evidence.
So, no answers I'm afraid but I do send you my sympathy.