Hey Sean,
If this is the first incident where you have found droppings etc on your car, would it not be worth waiting to see if it happens again first, before looking to surround your car with repellents on a daily basis?
I don't remember you ever mentioning this happening before, so it may be that this was just a one-off and may not happen again.
Might save you money and time on repellents if this is not likely to happen again.
I am inclined to think that it was a fox that randomly found its way into the parking area, rather than a cat that has just moved in to the underground parking area.