Question regarding the "smart" operation of keyless ignition/lock sensor

[xrunner]

I have a 2017 Subaru Crosstrek. It has keyless ignition and keyless entry. On the outside handle you touch it on a certain area and the car will lock or unlock - provided you have the key on your person (in your pocket), or even if someone else close by has them.

When I left the keys inside by accident (in a cup holder) and closed the door, and then touched the lock area of the handle, the car made an alert beep and the dashbord had a message "Remove key before exiting". Pretty nice.

Thinking about how it was able to tell that the key was not outside on my person, I first considered that it could sense very accurately the power level it was receiving from the key transmitter. I then put the key way in the back of the car which is farther away from the dashboard than I would be outside the front door, and tried to lock the door, but it still "knew" that the key was inside the vehicle.

My question is - how is it able to determine this?  Remember that you do not have to hold the key in your hand for it to lock/unlock outside the vehicle, the key simply has to be close to you such as in your pocket, so it can't be interacting with any capacitance of your body, at least not directly.

[xrunner]

Note: I did another experiment. I placed the key 2 feet away from me on a block of foam, outside the car. I then touched the lock area and it still was able to tell that the keys were outside the car.

[SeanB]

Simple, there is a keyless transponder inside the car for the ignition keyless start, and then each door also has a small antenna integrated into the plastic of the handle, connected to another transponder as well. The RSSI signal from the key being inside the car will be higher from the internal unit than from the shielded outside units, thus it is able to tell location of the key reasonably easily.

[xrunner]

Right, so that was my first thought - power levels. What I just did was to open the driver window. I held the key inside the car a few inches and it gave the warning. Even outside the window a few inches it gives the warning if held up high where the window glass would be. Only if it's low like in your hand, purse, or pocket does it not give a warning.

Now what happens if I leave my second key in the car and have another key outside the car, does it give a warning, or does it know that the driver has a key outside the car and it's OK to lock the doors? I have not tried that yet.

[Halcyon]

Now what happens if I leave my second key in the car and have another key outside the car, does it give a warning, or does it know that the driver has a key outside the car and it's OK to lock the doors? I have not tried that yet.

If it's anything like Volvo any keys left in the car are disabled/ignored.

[drussell]

Now what happens if I leave my second key in the car and have another key outside the car, does it give a warning, or does it know that the driver has a key outside the car and it's OK to lock the doors? I have not tried that yet.

It will be able to differentiate between the two "keys" because they have different IDs, but what the default behavior is under that condition, when it knows there are two keys, I cannot speculate... 

[Cyberdragon]

I would assume it won't lock the door with the internal button if there are any keys inside. But I would think you could override this by locking it with the second remote.

[Mr.B]

I have a 2016 Mazda CX-5 with the keyless magic.
It also has the "walk away lock" feature where you don't need to push the exterior door button to lock it.
If my wifes set of keys are in the car and mine in my pocket, then try to "walk away lock" the vehicle - it beeps 3 times and does not lock.
However, if you then press the car door external button, it will beep once and then lock.
So with the Mazda CX-5 you must deliberately lock the other set in the car.
Note: It will not let you lock the car if the only set it detects are inside the car, which makes perfect sense.

[Benta]

I had a BMW 3-series (E90) with keyless entry/start.
In that case, the key actually had two transponders, an active and a passive.

The active was for the door locks (touch the door handle or press a button), and the passive for starting the engine.
When you touch the door handle, the car sends a challenge to the active transponder, which then responds with the correct rolling code.
The passive transponder only works inside the cabin (Faraday cage) and apart from letting you start the engine, it tells the system whether the key is present or not.

Hope this helps.

[xrunner]

I left a key in the car, and then with the second key outside I tried to lock the door. Gave the same message - remove key before exiting. So there you go.

[Halcyon]

I left a key in the car, and then with the second key outside I tried to lock the door. Gave the same message - remove key before exiting. So there you go.

I'm not surprised, it's a Subaru. 

[Dr. Photon]

Most of these systems have multiple short-range transponders spread around the car. Some by the doors, some inside, and sometimes one in the trunk (so you can't lock your keys in the trunk). The way the system works in that each "transponder" is really just an LF (I think 120 KHz) RFID transmitter that interrogates the key. They key, when interrogated, will transmit a reply back to the car on 433 MHZ. The 433 MHz is the same radio link used for the remote lock/unlock buttons on the remote. The car usually only has a single 433MHz receiver; sometimes the same receiver is used for listening to the key's transmissions and the tire pressure monitoring sensors. Since the 433 MHZ transmission is longer range, they use the 120KHz RFID to get the short-range "proximitry" type effect.

Note that the key is very much active; the system will not work when the battery in the key dies. Luckily, most (all?) cars have a backup in that you can pull an emergency metal key out of the keyfob that will allow you to open the car door, but not start the engine. Somewhere inside the car, one of the transponders can activate/power the key via RFID if the key is close enough. On a Subaru BRZ, you just push the "start" button with the keyfob itself! This lets you start the car with a dead battery in the fob. Also note that the key is only needed to start the car (you can chunk it out the window and keep driving with only a message on the dash) since cutting the engine while driving may be dangerous.

[Bassman59]

I left a key in the car, and then with the second key outside I tried to lock the door. Gave the same message - remove key before exiting. So there you go.

I'm not surprised, it's a Subaru. 

A friend works in the service department of a Subaru dealership. A car was brought in. The new kid was assigned to deal with it. The problem was that the car refused to lock the doors. The kid had no idea how to fix the problem, so he asked my friend, who asked, “Was this car’s owner the woman who was in last week ordering a new key to replace one she lost?” “Yes.” “The lost key is probably under one of the front seats.”

It was.