i have a problem

Bear with me...this one is kinda random, even for me.

We have several stray cats in the neighborhood, one of whom has chosen to make his home in and around our yard. I'm a cat person, so this doesn't bother me at all. We don't leave food around, but he does drink from our sump pump hose during the winter. Resourceful little creature, he is. Anyhoo, my office is now in the basement, and during the afternoon, I hear him meowing and clawing at the window well covers. I don't know if he's looking for our cat or what, but it's somewhat odd. And two times in the last month, I've walked into the basement to be socked with the smell of cat urine spray. I'm convinced that this stray is peeing in/on our window well covers to mark his turf. And it STINKS!

Now, I know what you're thinking. You're saying to yourself, "Alison, YOU have a cat. It's YOUR cat who is spraying." But I really don't think it is, and here's why. 1. She's 8 years old and has never sprayed before. 2. She's a female, and it's much more rare for them to spray to mark their territory than a male. 3. The smell dissipates after a few hours...if it were inside and in the carpet, it wouldn't go away on its own. 4. It's right by the window (which is, unfortunately, the window next to my desk. Eww.)

Anyone have experience with an outdoor cat spraying at your house? I'm open to suggestions!

Thanks~