How did this thing end up in a guy's car?

Gohilla says: "I found this [see photo embed below] on the chassis of my car while I was working on the exhaust system. I know it wasn't there a month ago, because I was under it working on the suspension. A quick search tells me that it's an rfid tracker. What does this mean for me? What do I do?"

I found a tracker on my vehicle. I'm worried. What do I do?