Thieves steal old batteries out of cars. Who would want an old battery! No one...but now they absolutely know who's buying a brand new one. They come back a few days later for their new battery. I don't think the faceplate was stolen in the hopes that you'd throw the old head in the trash. Massive long-shot, to be polite. My first thought was that your face plate matched one that they were missing. They were already in your car - how long would it really take to get the whole thing. Could be just somebody fuckin' with you, too.