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It's my understanding that the TPMS have a unique ID code, sort of like that in an Ethernet adapter...i.e., unique to that sensor. To get the car to recognize it as one of its own, and not from some adjacent vehicle, you need to go through the reset procedure. Once that is done, it remembers those values, and will only respond to those sensors, not someone you may be passing on the highway.
On the BMW, at least my model, it takes a short distance before the computer gets a chance to 'hear' from each sensor and report it. It does not light the fault light until it has tried, and failed to find ALL of them. If it finds some, but fails to find all of them, it declares a system failure. If it never finds any, it tends to only light the light, not generate an error message.
The things have a battery in them to power the transmitter. They also have a motion sensor that shuts them down when the vehicle has stopped.
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