How Does RFID In A Keyfob Know When Its Inside/Outside Of A Car?

How would the RF proximity sensors in the car differentiate between the fob being outside the door or inside the car?

There is a big piece of steel that the signal has to get thru from the outside.  Wouldn’t that attenuation be enough?

Answer number two:

Maybe it also looks for the presence of a passive RFID return signal from the FOB, which is unlikely to be detected from outside.

I have a similar car and key fob. But in the case of my car, I don't think it "knows" whether the key is inside or outside of the car. It is only present, or not present.

If the key fob is present, the car will unlock the moment my thumb touches a sensor on one of the door handles. So technically I could lock the key in the car, but it's just going to unlock the moment I touch the handle.

It's also possible to start the car with the key outside of the car. I forget exactly what happens if you try to drive off without the key - I'll have to test it. I scared myself once when I drove out to a remote location to fly my drone. When I was ready to head home, I couldn't start the car. I thought the key was in the center console but it had been in my wife's purse which I had tossed in the back seat. Apparently it was out of range. Had I not discovered this, I would have been stuck in a canyon with no signal and very little likelihood of hitch hiking out of there.

Perhaps this tool can help you run some experiments?

Just a Pair of These $11 Radio Gadgets Can Steal A Car

There are multiple antennas for the fob. Depending on the vehicle, at least one in/near each of the front doors.  The system can detect the approximate fob location based on timings of the signal hitting the multiple antennas.  It doesn't have to be accurate down to the last inch, but it is does have to be pretty close to within a few inches.

The Nissans have an antenna in the center console and each of the exterior door handles and that helps if differentiate whether the fob is inside or outside the car.

And now I don't touch the wife's fob anymore, I use the spare if need be.  We don't need a repeat of that situation.

We rented a 2018 Chevy Malibu on a recent trip and it had the fob and push to start. It also had a button on the exterior door handles that would lock all of the doors as long as the keyfob was outside the car. Expectedly (to me anyways), my S/O would always bump that button closing the door after exiting the car. The problem is that the hotel we were staying at only had valet and I simply left the fob in the cup holder. The car would honk twice every time she hit the button, indicating that it refused to lock the fob inside the car. It was a handy way of locking the car, as long as you remembered to take the fob with you. 

The fob, or proximity key as it's known, has a radio pulse generator that sends out a signal that is received by several antennas in the car body. The secret is that there are several antennas. By comparing the signal level at each antenna, the computer is able to triangulate the position of the key.