Who or what?

My husband noticed the other day that there were unusual fingerprints on our van. Then, conveniently, our sliding van door started having problems. Yesterday, we almost weren't able to shut it. Now if I had a really cool airbrushed design on my car and someone scratched it off, I wouldn't care as much. But my door, something I use every single day to get my kids in and out, that's a pain in the neck. I have no idea what the problem is other than I don't dare open it because I'm afraid it's going to be stuck that way. We called about it and were told to lube it and clear any debris. I'm praying that will work otherwise we'll need to come up with money to fix the door. Do you think that's expensive? I'm not sure I even want to know.