I saw him do it. I think he meant for me to see. I think he's frustrated by
not being able to spray outdoors like he used to, and at the same time he
knows it's inappropriate to spray in the house. So he found a compromise. "I
can spray in the shower stall because it doesn't hurt anything." I've
noticed other cats in the shower stall sniffing that spot.

I sure hope it's enough to keep him happy.