Old cat won't toilet outside?

[mushypeasnmintsauce]

Our old cat loves going outside still, has good mobility and all his senses. He can spend all day outside with us but only goes to the toilet in his litter tray when he comes back in the house. He used to toilet outside.... any ideas why he won't go outside to the toilet?

Maybe he feels less exposed indoors

He’s getting on a bit, home comforts are more appealing now.