All the books and vets tell you that cats cover their poo.
Sev has an oversized litter tray in his own bathroom - I have just seen him poo - by accident, I am not a poo watcher:
He steps on the rim of the tray, does his business, and then carefully touches the box sides next to him with his paw moving it ever so slightly. DD tells me he always does this.
Now I wonder - is covering genetic or learned behaviour? He was with his mother until he was three months old.