The ones I can think of that would be good for in car would be pretty pricey to risk being stolen themselves. However, you might be able to find a small 5v cam

The ones I can think of that would be good for in car would be pretty pricey to risk being stolen themselves. However, you might be able to find a small 5v camera (and use an adapter from the cars accessory port) that will capture stills on motion for a reasonable price, maybe try tiger direct or monoprice, newegg etc. Then just hide it reasonably well in the car, rear trunk deck or above the rearview mirror would be ideas -- use colored tape to make it blend into the interior. To transmit the image as well as store local that is a little more involved. Outside of something like a nest camera, you could use a RaspberriPi with camera and have it connected via wifi to your house so you could get the images. It would be a little tougher to conceal, but still very doable. Obviously I think just locking your doors (if you don't already) is the smartest idea. And setup a camera outside, at least that way you can start to deter this behavior and possibly catch a glimpse if it does happen. I'd also bet if this is happening multiple times, it is someone you know, or a local teenager etc. That's what happened to me when I found my car getting egged etc repeatedly one summer, I setup a system to catch them which allowed me to have a little discussion with them to make it stop before it escalated. A little fun too, if you only use the outside camera, put a sign in the car that says "don't look back" or something to that affect (to get them to look towards the camera). Most people when they read that immediately do the behavior you wrote to see why, and they'll look right at the camera. Only the real criminals will just bolt and not do it.