If they are like Hertz, they may not always have someone handle arriving cars right away. I am often told they would deal with it shortly and email me the receipt.

I wonder how long it took them to figure out where the car belonged. The OP said “After 4 weeks Hertz emailed me that the car had been located at the Avis maintenance garage”, but I can’t imagine that Avis kept the vehicle for four weeks. More likely that it took Hertz that long to get back to the OP.