US Parking

If the J church is unreliable, I’ll drive to work or carpool with other friends or family that work downtown. I often park here because it is very close to my office. However, if you arrive late (around 9am), sometimes the lot is full and they do not allow you to self-​park. Instead, they ask for your car keys and they valet it.
If you park your car yourself, it’s great. Plenty of parking spots make it easy to park without being on top of another vehicle. If they request your key, it’s a completely different story.
Three times I’ve left my key. Three times I’ve discovered that someone has been lounging in my car. Once, upon returning to my vehicle at the end of the day, there was mysterious garbage in the backseat. A second time, the radio station had been changed and it was NOT one of my own pre-​sets. And then last week, there was a used teabag and it’s packaging left in the pocket of my passenger door. I never drink beverages in my car and even if I did, it would not have been hot tea. I also have never used that brand of tea before.
I am absolutely livid that not only are they using cars as their own living rooms and kitchens, but they are so careless as to leave garbage in the vehicle and turning it on, wasting battery life and possibly gas. This is clearly a breach of contract and I’ve got to find a way to prove it. Believe me, I will be calling this morning at 9am to complain to the main office.
If any of you park here and leave your key, I suggest checking on it now and then and bring a camera with you to catch them in the act of violating your privacy by hanging out in your car. Do I smell lawsuit?