Something like this happened to me - I'd rented one of their cars with no return date because I hadn't fully planned out a multi-week trip in the eastern USA. The staff member said this was fine; they'd put in a return date of a week or two and then keep pushing it back.
Five weeks later I returned the car and flew back home to Australia. Found letters that had been posted saying that the car hadn't been returned and was considered stolen.
For some of these cases, it was arguably worse. It turns out that when Hertz gets back a vehicle they'd reported as stolen, they might "forget" to tell the cops that.
They'd rented a car for a fixed period, and were arrested and jailed for driving a stolen vehicle. They've shown police the lease agreement from Hertz showing they were authorised to drive the vehicle but police either couldn't or wouldn't verify the information with Hertz.
If being arrested and jailed for this wasn't bad enough, in some cases Hertz managers were slow to respond to police requests for information - so people have sat in jail for extended periods.
Five weeks later I returned the car and flew back home to Australia. Found letters that had been posted saying that the car hadn't been returned and was considered stolen.