It's Hertz.

They've either completely forgotten that you ever rented it....or they think you boosted it and the police are on their way to your house right now.

You never can tell...

A few years back, I had a 1-day award rental at DEN. Rented and returned, no one outside to check it in, so I just got on the bus and left. Three days later, I get the "where's our car?" phone call. I suggested they look around their lot in Denver for it. Thankfully, they called back a half-hour later saying they'd found it.