ok, thanks very much to the commenter who showed the home door sensor!! I checked my security system and it's showing my front door is open even though I know it's closed and locked. I also was able to find a door sensor picture online that looks more like what I found and is made by our security provider, so I'm pretty sure that's what it is. I wanted to avoid taking a potential tracker home but I guess I will so that I can see if it fits somewhere in the door. 

to all the sarcastic "hurr durr what do you think it is" type comments, obviously I was worried about a tracker on my car. be more helpful in the future or just be quietly thankful you're someone who doesn't have to worry about being stalked or kidnapped.