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!