We've had a problem with a tom coming in through the cat flap and spraying the last week.
I find a few drops here and there and although there's not much of it it really stinks. Final straw was today when he pissed over my coat which I'd left hanging off the bottom of the stairs to dry off after getting caught in the rain.
Tonight I trapped the prime suspect by putting the cat flap on in only. My three peeped from the stairs and didn't do anything. I threw 3 pints of water over him before letting him out. He growled and hissed at me but hope it's put him off a bit.