My story, again: Was getting off the freeway and saw a movement out of the corner of my eye: down in the lower right corner of the windshield: for a couple of seconds, I saw a little wiggly line extending into the air, which quivered out of sight below the windshield. "Holy S***!" I practically screamed, "A mouse!" My wife looked up just in time to see the tail disappear too. Well, the instant we stopped (a few minutes later at a wedding reception), I was all over my engineroom with a flashlight: saw some bits of leaves and whatnot beside the fuse box. When I checked the air filter later (didn't know about that at the time), it was "clean", as in only normal bits of stuff, nothing mouse-related. I laid traps and got nothing. Decided the little bugger had gotten the scare of its life and vamoosed the instant the car came to a stop.
I've not seen any trace of mouses since the disappearing tail: I've checked my air filter several times since. And yes, I attract kitty cats into my yard by laying out food. Three, four times now (as recently as last week), I've seen kitty paw prints on my car; so, if there is any mouse, it's dead meat. Hah.

I have at least four different cats vying for the Alley Cat that I put out each day. Not enough to feed them, just enough to entice them. Here are two of them, "50/50" and "Bibs" (my names):
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OP, be glad you didn't find dead baby mice laid out on your filter. Someone here posted about that after my episode; I nearly had kittens reading it. Another poster showed pics of rubber and insulation in the engineroom all chewed by rats. RATS! The mind boggling quality of that picture is beyond the pale.