I've had this happen. The worst case there was nobody working returns other than someone from Enterprise shouting to just leave the keys in the cupholder, so left the car. Never got a receipt. Two days later, get one email saying rental is overdue and then been extended on my card. Card gets hit. Happened the next day again. I call customer service (after trying to call the local airport branch where I left the car) and they said they'll send a message to the station, and they can't get anyone on the phone either. I get another overdue notice and extension on my card. At this point I at least want a receipt to do my expense report.

I got it resolved. I did finally get someone local on the phone. I had one of my employees go and try to resolve it who was local but they wouldn't assist them at the counter. So (and full disclosure: I was Regional Manager for one of the airlines flying to this airport) I called up the airport authority operations center and they said they would get me a call back within 15 minutes. The GM called me just under 15 minutes later. He didn't know what the issue was he was to address, but told me that an airport commission police Lieutenant came into his office, handed him a cell phone and said to dial my number right now. Whole rental got comped (I only really had the car for about 36 hours).