A couple of years ago my Mercedes 240D was hit with an egg while parked in front of a friend's house, right on US rt. 50 in eastern WV. Luckily, I discovered the attack later in the evening and was able to get most of it off before the the sun had a chance to bake it. As a result, there was little or no paint damage, but it sure was a mess to clean. The egg hit the driver's door window, right in the middle, and from the extent of the splattering, and how deeply it was impacted between the glass and the rubber gasket, I would guess it had been thrown from a moving car. I was still finding bits of egg here and there (all the way at the back of the trunk lid !?!) for weeks, and it took quite a while for the smell to go away. Good thing the window was closed. Of course, I had no way of tracking down the culprits.

It sounds like you have taken suitable measures to try to catch the troublemakers. Hopefully the police will be willing to act on any evidence you may be able to provide them. Maybe they have been vandalizing the cars they can't steal gas from.