Ohh you guys, lol
Hmmm, install a tied up bag o cat nip somewhere under the hood, or a glue mouse trap with peanut butter, that is a tough one to choose from, if I had cats that is, so I guess glue traps it is.
Michael'* point of them dying up inside of the car body somewhere is a great point, I did not think about that one, thanks.

I opened up the airbox and it does look like something moved in, only a tiny piece of something looking like bedding, and a few nuts, it was unsatisfying to find, but at least for once I was not the only nut under the hood, lol
I have not gotten any screen yet, but I will do that ASAP, as I pretty much live in what you described Michael, and there are lots of moles, mice, etc...

moth balls to keep them away. and some fly poison pellets elsewhere. i left about 4 pellets for a coon up under my house, and he didnt make it ten feet before he dropped dead. i weighed him. he was 35lbs

I put a whole large bag of moth balls in and around my car when it sat in storage for two years. The mice still made a nest in the air filter for the HVAC and nibbled on the insides of my seats. Plus my car smelled like moth balls.

If I had to do it over again I think I would buy a big tarp, drive the car onto it, then wrap it over the car and tie it at the top. Or get one of those zip up bag things. I think they exist... If not someone needs to invent one ASAP.

I'd have to find somewhere under the hood to put a bag of mothballs, and hope it would help deter them from coming back.
I'm trying to find something like some window screen, and find a way to attach it over the opening to my air-box, that would at least keep me from feeling like I have to constantly open the air-box up to check it, I am sure they will come back to that same area like they did with you Tom, so I'll have to remember to check every once and a while to see if they are getting in there.
I always open my hood before driving to remove my battery maintainer, so maybe I can just keep a trap, and or moth balls under there/on the air-box at all times.
I mainly worry they will end up nibbling on some wires, or EVAP tubing, and I'll be left tracing down electrical issues, which are a pain...

I am glad I have never removed the screen in the throttle body like some people do, else I bet I'd have mice in my upper intake manifold, then again the smell of oil up in there may keep them from venturing that far in..

I have heard Bounce fabric softener sheets are also a mouse deterent, without the odor of mothballs. Any signs of mice in the interior? If so, keep the visors down. They like to nest between the visors and the headliner. I would set traps around the car, and fabric softener sheets (or mothballs) all over the engine bay, along with covering any openings into the engine (intake, exhaust, etc.). I've also heard of putting little bottle caps filled with engine antifreeze around the car, but you can't do that if you have pets.

Thanks for the tips guys.

I have seen no evidence of mice inside the interior, though I'm sure they are looking for another spot now since I temp have my air-box out, as I am looking for some screen and a way to keep in on the air-box opening.
I keep my visors down to help hold up my sunshade, I will keep an eye out for any signs of them getting inside the interior.
I have pets, and my landlord does as well, and he passes by my driveway daily, so any non-safe for pets methods are a no-go for me.

One more thought. If your car is an a garage keep the hood wide open when parked, and if possible, shine a bright light on the engine compartment. Mice look for dark spots to nest and will avoid well lit areas. I used this approach to get rid of skunks under my deck. I put a shop light under there (300 W) and kept it turned on 24 hours a day. The skunks cleared out.

this?

^^^ Brilliant! lol
Over my budget, and not available in the US though, well darn.
Outdoor Carcoon

With squirrels & mice running around the boat & Jeep, I use mothballs under the vehicles and use a container size like the international coffee`* with holes cut in plastic top & 20 mothballs inside, leave it under the hood of jeep & under boat etc.
Keeps critters at bay & I no longer find squirrel nuts under hood etc.