This past Sunday I had a slightly distressing experience with my Model S. I drove up to a hiking trail near San Francisco and parked in a parking lot at the top of the mountain where there also happened to be several nearby cell towers. To my surprise, when I returned to my car I could not open the vehicle with the key. Pressing the buttons and moving near the car didn't work. Even putting the key on the windshield near the passenger side windshield wiper didn't work (which in retrospect makes sense since the issue was wireless interference and not a low key battery). Eventually I used the key to unlock the doors by holding it inches from the outside of the rear windshield glass. I didn't think about using the mobile app at the time but that would have probably worked too.
I then got in the car and tried to start it. No matter where I put the key in the interior of the vehicle the car would not recognize the key to allow me to start the car. I was on the phone with Tesla roadside assistance (who were very helpful) and they had me move the key to every inch of the interior including putting the key on top of my parcel shelf to no avail. The key would not be recognized and I could not start the car by putting my foot on the brake pedal. Thankfully we eventually discovered that if you put the key on the OUTSIDE of the rear windshield at the base of the glass, the car DID recognize the key and someone sitting in the driver's seat could press the brake and start the car. However I also discovered that if you leave the key on the rear windshield where it would be recognized and start the car by pressing the brake, the moment you get out of the driver's seat to retrieve your key before driving off, the car turn's off again! Thankfully I had a passenger with me that could retrieve the key while I stayed in the drivers seat after starting the car.
If the car had a passcode protected ignition, accessible on the touchscreen, that would have made the ordeal much less trouble and would also ensure that someone traveling alone would be able to start the car in the same situation I was in. If any wireless engineers in the Bay Area want to do some testing, I'd recommend going to the address below where the cell tower interference caused my issue.
900 Radio Rd
Brisbane, CA 94005
All in all, this problem I experienced would have affected any car's keyless entry and ignition system I'm sure, but Model S is unique (as far as cars I've driven) in that there is not really a convenient backup method for starting the car. My key has worked flawlessly since I left that specific location so I'm very sure that the issue was wireless interference and not a low battery in the key. I now remember from a previous trip to that location in my BMW that the keyless entry system also didn't work but my BMW had a physical key built into the FOB as a backup. Anyway, keep the rear windshield trick in mind in case you're ever stuck because of key signal issues