With 185 rental days and counting this year I had my fair share of these kinds of situations. I made fully resetting all electronic systems in the car as much

With 185 rental days and counting this year I had my fair share of these kinds of situations. I made fully resetting all electronic systems in the car as much as possible (which sometimes ends up being not at all) a ritual before returning cars. Some cars offer a master reset (such as Ford with their SYNC system), some don't (such as BMW's iDrive). Some cars can have their multimedia systems reset but retain some information about the paired phones and some history as fragments on their SD card (such as the Nissan Maxima, where the multimedia system also malfunctions by forgetting all settings between power cycles if no SD card is present, make sure you check before you pick one up). But in 95% of cases people don't give a damn and I would get a car that has all the previous customers' data in it. Phone logs, Navigation history, sometimes even contacts and text messages. I think it's scary.