Thanks, everyone, for your responses. I called the vet yesterday afternoon and Hunter was checked out and given a clean bill of health. His vet thinks he's gone into a phase of marking territory despite being neutered. Yikes!
They recommended a pheromone spray that's supposed to discourage the spraying. I hope it works. I've always used plastic liners in his litter box and he's never minded in the past (he'll even pull it up on the sides and use it to cover his "work".)

Hunter and Chloe and still like two strange peas in a pod. Both of them get individual "mommy" time for playing, grooming and belly rubs . Our routines haven't changed. However, earlier this morning I started thinking again about it and wondered: since the weather has been much warmer the windows have been open. The window with the (now stained) curtain faces the back yard. We get our fair share of wandering cats coming through and I wonder if he's reacting to their presence. In any case, I don't want the situation to get out of hand where the landlord says he has to go.