Isn't this always a fun mystery to solve

Hmmm...tough question. Is this on a door inside the house that leads to the garage? Or is this a door outside...going into the garage?
Could it possibly be that there is an outdoor kitty that got to close to your home, and one of your boys thought it would be a good idea to let the strange kitty to know that your residence is taken? LOL

Has anything changed lately? It could be anything at all from a piece of furniture being moved to another animal to new litter. Sometimes a cat will get upset and will spray.

As for cleaning, use an enzymatic cleaner. Cats will tend to go to the bathroom where they can smell they've gone before, so it would definitely be best to try to get the smell out.

The Feliway sounds like it would be worth a try, and in fact I think I'm going to give it a try just to ease our skittish Scooter's behavior. Noahsmommy uses it and recommenede me to try it out. It's worth a shot!

Good luck!
**hugs**
Karen