I’m providing some follow up advice. A year or so ago, when planning a trip to Switzerland, using public transportation, followed by France, using rental cars, I wanted to book our cars at a car rental facility on the French side of the Geneva airport. The cars were going to be returned in Avignon, France and I wanted to avoid the multi-country rates. I posted a question on this forum because I was confused about how to get to the car rental desks on the French side of the Geneva airport, when arriving at the airport by train on the Swiss side. It appeared to me that at one time, the airport had allowed pedestrians to walk through the checkpoint from the Swiss side to the French sector so long as they possessed either a boarding pass for a flight OR a confirmation of a car rental, but then the rule changed and a flight boarding pass was necessary. I was advised that since we’d be pedestrians without a boarding pass, we’d be able to see where we needed to go, but would not be permitted to walk through that checkpoint. Instead, we’d need to catch a cab and have it drive us through a tunnel and around the airport, depositing us in front of the French sector, a few hundred feet from where we had started, with the cab being a possible 30 Swiss Francs or so. That roadway was not useable by pedestrians.
We were traveling as a group of 10 friends, with three cars booked, so we’d need to pay for multiple cabs or be sure we could send the three drivers by cab and have a way for them to pick up the rest of us without having to purchase a Swiss vignette (40 Swiss Francs) for each rental car. I emailed the Avis facility on the French side just prior to our trip and they confirmed that we wouldn’t be able to walk to the rental desks on the French side but would need to take a cab; however, no vignette would be necessary for the cars to drive around to the Swiss side of the airport to pick up passengers.
When we arrived at the Geneva airport by train, there wasn't obvious signage directing us where to go, so I asked at an information desk. The person there told us how to get to the France "checkpoint" and again confirmed that once there, we'd need to get a cab. When we got to the manned checkpoint, I told the guard there that we needed to pick up our reserved rental cars on the French side, and I asked if the visible exit door was where we’d be able to catch a cab. The very nice guard said, “just pass through,” and he pushed a button to let our whole group through the locked door there. The car rental desks were just on the other side. I think we just got lucky and caught a guard on a good day when he decided to do us a favor. Maybe because I used French to ask him if he spoke English, he was being very nice to me!
In general, assume you’ll need a cab if you’re doing what we were doing, but maybe you’ll get lucky like we did.