A car's immobiliser keeps a list of the transponder keys it will start for, and a programming tool reads the list and says which key is present. This car lists three. Two exist, one on the owner's ring and one in her drawer. The tool reads the third as present, in the ignition, every time the car is in the workshop, with the ignition empty.
I am the technician who does key programming at a main dealer, which is a job with paperwork because of what a car key is worth to the wrong person.
How the register works.
The immobiliser holds a list of transponder IDs, up to eight on this make. Programming a new key means the tool, the security code from the manufacturer, and a log entry, because the manufacturer wants to know who added a key to which car. The tool can read the list and it shows which key is currently in the ignition, by its ID, because the car reads the transponder every time it starts.
Keys on the list that are not in the car show as registered. The one in the ignition shows as present.
One car has three keys on its list.
The owner has two. The dealer record shows two supplied with the car in 2016 and none programmed since, and I have checked with the manufacturer's log, which shows the same.
The third ID reads as present. Every time the car is in the workshop, with the ignition empty, the tool says key three is in the car.
A key that reads as present is a transponder within a few centimetres of the ignition barrel, being read by the coil round it. That is a physical thing at a physical place.
There is nothing in the barrel. I have had the steering column shroud off and the barrel out.
What has been checked.
The tool. Two tools, two makes, both read three keys, key three present.
The immobiliser unit. Replaced under warranty, which required all keys to be re-programmed. I programmed her two. The new unit's list, on the next visit, had three, and key three was present.
The car. Coil, wiring, barrel, all replaced with the unit.
The owner. Divorced, husband had a key, husband returned it at the time and it is the one in her drawer. Nobody else has had access to the car. She is not a person who leaves it unlocked.
The ID.
A transponder ID is a number, and the manufacturer can tell you from the number when and where a key was made, because they make them.
I asked. Key three's ID is in their format and their range, and their database has no record of a key with that ID ever having been manufactured, cut, or supplied. It is a valid number for a key that was never made.
The car has been reading it in the ignition for years.
What it does.
Nothing, while the owner drives. Her key is in the barrel, the car reads her key, key three is registered and present, and the car starts. Two keys present at once is not a state the system is designed for and it appears not to mind.
What it does when her key is not in the barrel is the part I have not put in a job card.
With the ignition empty and the car in my bay, the dashboard warning for the immobiliser, which lights when no valid key is read, does not light. The car believes it has a key in it. And on two occasions, with the bonnet up and me under it, the starter has turned. Not started, because the fuel is cut with the bonnet open on this model. Turned, for a second, the way it turns when somebody in the seat twists a key.
What is done about it.
The car is serviced and it is sound. I have written the three key register on the job card as customer to be advised, and I have advised her, in the sense that I said the car had a spare key registered that she did not have, and asked whether she wanted it removed.
She said she would think about it. That was two years ago.
What I do.
I service her car when it comes in.
The one thing I have changed is that I do not work under the bonnet of that car with the driver's door shut.
With the door open I can see the seat and the barrel from under the bonnet, and I would rather be able to, because on the second occasion the starter turned I came up from under the bonnet fast enough to bang my head on it, and the driver's door was shut, which I had not left it, and the seat was pushed back further than she has it, further than anybody her height would have it, and the barrel was empty.