I never leave my bike in the car overnight.

I cover the bike with a blanket that is the same color as my car interior, so you can see there's something lumpy in there but you don't know what it is.

The first year I had the car, I had a vinyl shower curtain under the bike to protect the car from dirt. After a while the shower curtain got some holes in it, so I tossed it and since then I don't put anything down. I haven't had a problem since then with dirt inside the car.

The blanket that covers the bike, however, has plenty of grease stains on it.