TBH, the way their systems and inventory are managed, you could probably park the car in an out-of-the way stall in the Hertz lot, leave the keys in it, walk away, and no one would ever be the wiser--and you'd pay the one less day. Eventually an inventory report would identify the car as having not moved in a while and someone will go out on the lot and look for it and find it and process it for cleaning.

That said, doing a normal return is the best and most honest/ethical thing to do. They may be able to reopen your contract and then close it with the correct time, but I don't know if Odyssey (National's computer system) supports that functionality, and generally, once the calendar day has ended, stuff from that previous day is posted to the accounting system's general ledger and is no longer changeable. A manager may either manually add an adjustment for the extra day to your existing rental (as a post-close adjustment) or they'll zero out your contract and recreate a duplicate with the correct information. (I used to do the latter.)

Feel free to share your experience afterwards...I'm curious how National handles it.