So I actually tested it the other day. At first, I stepped out but didn’t close the door, and it did not beep or anything. When I closed the door, it beeped 3x (not terribly loudly) and the dash flashed a message, “Ignition key not located” or something. But nothing more. With the way the damn car beeps and flashes messages all the time, it would have been easy to miss.
I walked at least 20-30 feet away, and it just kept running.
Seems odd to me.
But did it keep running that way more than a few minutes? Because all my cars with keyless entry/pushbutton start will run without a key present for up to about 10 minutes, but shut off promptly after that.
Even the 11-year old Altima.
These last couple of posts bring up a good point.
If you have a fob car, it’s worthwhile to really learn by experimentation what it does if you goof. e.g. Start the car, get out, and walk away with the fob in your pocket. Start the car, leave the fob in it, get out, and walk away. Ensure you have two fobs with you, put one far enough away (50 feet?) that the car definitely doesn’t sense it, then put the other fob in the trunk and close the lid.
By experimentation (not all of it deliberate 
) …
My own car is smart enough it refuses to lock from the outside if there’s a fob inside. It won’t close the trunk lid if there’s a fob in the trunk. It won’t lock itself if the engine is running. But you can lock it manually. It will fuss if all the fobs walk away with the engine running.
My objection to fobs is they’re huge. I want one about the size and weight of a dime; not 2" x 1" x 1/2" weighing what seems like a pound. I don’t need any of the buttons; I just want the RFID thingie.
In the current state of the art, it’s not passive RFID but an active RF transceiver with or without buttons, so requires a battery, and the size of the battery sets the size and battery life of the fob. All the ones I’ve seen use a CR20xx 20-millimeter coin battery, so you’re not getting much smaller than 1" diameter (or square).
Smaller batteries would allow a smaller fob, but require more frequent battery charges.
Also, most fobs I have seen incorporate the “my car battery is dead” emergency mechanical key, so you would be doing without that or putting it on a keychain anyway.
Mine does this as well, but I discovered the “DetermineFobLocation” process runs in a slower rate group than “RespondToDoorLockButton”.
In one of my rare trips into the city I was confronted by an aggressive homeless guy. He kept trying to get between me and the driver’s door, determined to exact some coin from me. I feinted back and forth and fooled him into running partway around the truck. Then pressed the fob button once (in my pocket) which opened only the driver’s door and leaped inside. The instant the door was closed I punched the lock button. The “system” hadn’t processed the fob’s new location yet – so it sounded an alarm and unlocked ALL the doors.
My truck does something similar and it’s absolutely infuriating. If I get out with the fob in my pocket and close the door, it sounds the horn loudly (HONK - HONK - HONK - HONK) four times.
I’m frequently in and out while positioning our camper in RV parks and storage lots, and I’m subjected to this warning if I close the door for the occupants (either climate or keeping my dog inside). I hate this noise bothering everyone in quiet parks, and I hate the loud announcement in some areas of “Hey Everyone! The driver just left this vehicle and it’s running – Now’s your chance!” I absolutely loath this feature, since it’s a large IC engine and far from quiet. There’s zero chance of me wandering off and leaving it running.
And car thieves love it even more. Here is an article about Toronto’s car theft problem, it’s doubled in the past few years, apparently due to the ease of relay attacks on fob-based ignitions with keyless entry.
Dallas also hit record highs in the past few years with thefts, and DFW airport car thefts doubled, ostensibly for the same reason – it’s absurdly easy to steal a keyless-entry/push-to-start vehicle with the correct equipment.
Personally, I decided to head off all the above problems by modifying my truck to make it safer and more usable:
Frankly, a hell of an effort to return to the usability I had before this electronic wizardry “improved” my life.
I let a friend test-drive my new car once. I gave him the fob, even though I was in the passenger seat next to him. When we got to his place, he stepped out, and I got in the driver’s seat and left. I was a few blocks away before the car started warning me that the fob was not present. That’s when I got a bit nervous: the car had a manual transmission, I was still getting used to the clutch, and I knew if I stalled it somewhere - potentially right in the middle of traffic - I wouldn’t be able to restart it. Fortunately I didn’t stall the engine, and managed to navigate back to my friend (and my fob) without incident.
A week or two ago I was going to posit a theoretical scenario about another person somehow walking away with the key fob and me driving off without it. Then I thought, nah, too far out even for this thread. But Machine Elf’s and Pullin’s posts really make the overall point well - the complexity is not nearly worth the benefits, IMHO.
Frankly, I have no idea how my car would behave in the harassment situation Pullin described. Also no idea what happens if I drive away without the fob; this latter scenario again being completely obviated with a keyed ignition system. Guess I’ll have to read the manual - thank goodness companies put so much effort into clear and readable documentation. 
Llama F. Buckley - Technological Conservative