This is probably an odd question, but here goes..
I have two rescue cats, both 14, one male, one female. They're indoor cats.
Mrs Cat uses the litter tray regularly, I have just put the lids on the trays, having started out with no lid, just to get them used to where the trays were and make sure they'd use them.
I have never, ever seen Mr Cat use the tray. There's no mess in the house so he obviously does!
I have one of those Smartfrog cameras, and moved it to the kitchen where the trays are (of course - why would I put them in the bathroom..!), and it has motion recording. Over 4 days I've seen her use the trays numerous times, but never him - not once! There is defo poo for two, but how is he doing this so stealth!
OR has he learned to use the toilet and flush..?!!