In response to pibrahim's post above and for folks who are renting a car elsewhere and returning their car to the French side of the Geneva airport, the information I received below this past Monday 7/24/2023 from Enterprise (along with a number of YouTube videos my husband found online) is very helpful in getting to the correct place. (Enterprise sent this to us the morning of the day our car was to be returned; it would've been helpful to have this sooner!) The circle on the map below indicates the roundabout you'll encounter just after you pass through Customs; it's really just a small jumble of plastic barriers, which you go around counterclockwise in order to make a sharp left turn onto the road that's marked here as "Aéroport Secteur France". The roofs visible in the photo below are of the customs buildings; these are NOT the rental car agency locations!! You have to drive about 2.5km/4 minutes further after the roundabout to get back to the airport, and then enter the garage where the car rental return area is located.

We did encounter some traffic lining up for Customs/Douane (which added an extra 10 minutes or so to our original ETA), but once we got to Customs we were just waved on; the guys didn't seem particularly interested in asking questions or checking out the contents of people's cars. Nevertheless, I'd suggest leaving a buffer of time in case it is more crowded due to weekend traffic/travelers; we were there on a weekday evening around 7pm.

We dropped our car off the evening before our flight back to the US and stayed at the Crowne Plaza Geneva (Airport) hotel before our flight out the next morning. We were able to walk from the French side of the airport to the Swiss side (from *within* the airport) after dropping the car off. Just follow the overhead signs from the French side that say Suisse/Switzerland with the Swiss flag. You'll pass by some duty free shops on the way; it's not just an empty corridor. At the point where we crossed into the Swiss side, we were just asked by an official to show our boarding passes for our departing flight. The official had no problem with it being a +1 day flight, so the rules do seem to allow for this. I did have a printed copy of my original rental agreement with me, just in case.

Incidentally, we were rather shocked that we did not receive any sort of hard copy receipt/return confirmation at Enterprise after turning in the car; we were told that "everything is by email". We still hadn't received the email later that evening, so considered going back over to the French side in the morning before our flight to get a copy of the contract to prove we'd returned the car. We asked an airport staff member if it is possible to go over to the French side and then return back to the Swiss side, and she said yes, as long as we have our boarding passes with us. Ultimately since we were short on time, we just decided to call the French Enterprise office in the morning (using the phone number in the image below), and they said they would send us a copy of our completed contract, which they did. But it was rather unnerving waiting to get confirmation of the final cost we'd been given orally at the car return location (in the garage, not upstairs at the Enterprise desk). YMMV--I'd suggest requesting a hard copy from the rental desk upstairs after you return the car. Note also that if there isn't an Enterprise staff member in the garage when you arrive, you can use the telephone on the wall near the Enterprise parking spaces, which connects directly to the desk upstairs, and someone will come down to meet you.

Apologies for the lengthy post, but am hopeful this information will help others reach the French rental car return location successfully!