
It's just possible that, if you clear up after him very quickly, he may think that you don't like to see the poo, and so he "removes the evidence". Not that I'm saying you've ever scolded him for pooing, perish the thought!, just that he may have got a little confused.
I had a very geriatric bitch a couple of years ago, who became incontinent in her bowels, and she would poo without knowing it, often in her sleep. If she woke and saw it on the sofa where she slept, she would "clear it away" (yuck!). And she was never scolded either. She just knew it shouldn't be there.
Leaving it down for a little longer might, if this is the reason he's doing it (and I've found labradors as a rule like to help!) let him relax about it and learn to ignore it.
Good luck!