Foxes, if I remember correctly, are related to dogs. Which is to say that they're not the most paranoid of animals.
I've had a few running around my parents yard. The most recent 'invasion' has been under their neighbors shed. He called animal control and was told flat out that he wasn't allowed to touch them. They are a protected species in PA, apparently, so you might want to check the local laws before going out guns blazing.
As far as them being rabid for being curious about the car, I wouldn't sweat it. The neighbors fox has no problems raiding my fathers garbage cans (I helped him lug one of the winter tires on top of it which solved the problem) and it loves the birdfeeder both for the feed berries -- and the birds. While they are nocturnal, it's more in the sense that most animals are which is to say they're active in the morning (shortly before sun-up) and evening (shortly after sundown). So basically there was a loud smashing about the cans at the most annoying hours you could imagine and my mother threatened to invade my apartment for some quiet.
I have had foxes pace me while hiking and geocaching, remember, they think you're fantastic for kicking up all those juicy McMice. They seem to regard people as walking buffet lines. I have run across a few rabid animals while hunting, and it's generally pretty easy to spot the loping, aimless walk. But my guess would be your fox died of antifreeze poisoning.
Just don't tell the neighbors if they ask where there cats went. I need to use the driveway too...