I am currently going through this in Avis AUS (Austin) airport location. We arrived at the AVIS return line and Avis personal asked us to park the van and leave the keys in the van. We did like he asked and left off to catch our flight without taking pictures.
Despite complying, I am receiving notifications that the vehicle hasn't been returned, and my credit card is showing multiple pending charges. Called them(waited for ~20mins) and now they assigned me a case that I need to follow up after 5 days. Wow, what a hassle.