If it was actually on the key ring, I doubt it made it to the next renter. The keyring would actually be one of the more obvious places it would be noticed by a Hertz employee, since they use the key tag to look up the vehicle number, mark on the key tag where the car is parked and how much fuel is in it, etc. The risk is if it's flat and non-obvious that it's of value, someone might have yanked them off and thrown them away, but I'd guess that Hertz's procedures would be for anything on the key ring to be dropped off at Lost & Found. It just takes them several days to categorize it, look up what contract it was on, etc.

If it was returned this morning, it may or may not have even been cleaned yet. If it has been cleaned, the L&F stuff for the day may not even get delivered to the L&F office until tonight or tomorrow, and then it may take them another couple of days to sort through the L&F bucket and categorize everything and look up what the last rental agreement was and all of that. Given all that, I'd think it might not (if it indeed was found and turned in) be posted to the LostReturns website by the end of the week.

Unless you make a lot of squeaky-wheel noise, I don't think anyone at the ATL L&F department will go through the trouble of actually going out to the car and digging through it or even looking up to see if the car is back out on rent or anything. I think their procedures are basically to let the detailers clean the car and turn stuff in, and then they go through the stuff that was turned in and store it until it's claimed or donated. Note the L&F option on the phone tree usually diverts to a central call center, so it may take some work to get in touch with the L&F people at the ATL location itself, and even then, it's likely pretty hard to motivate them to do the extra work to either look in the car (if it's still there) or tell you what the current status of the car is (if it's back out on rent).

Crossing fingers for you, though.