Access Control Qt: Ideas For A Low Cost Way To Detect Proximity To Specific Vehicles?

I have a customer using a driveway entrance alert system that triggers an alert inside the building whenever a vehicle crosses a magnetic field sensor. It's a simple system that can be enabled / disabled by relay control.

They have 4 specific vehicles (company owned) that should NOT trigger the alert when they pass over. This is a low traffic entrance / low-security entrace so price is more important that eliminating all false-positives / false-negatives. It should work most of the time (most company vehicles should be ignored, most non-company vehicles should be alerted).

I was thinking it would be straightforward to find or build a system that used a short-range RF transmitter in each vehicle for this - but after a few hours of searching I can't find anything for a reasonable price (< $500 or so). Ideally, I'd like to mount simple ignition-powered transmitters in the vehicles, and a receiver close to the driveway sensor. Whenever the transmitter was close by, the relay would open and remain open until no other active transmitters were detected.

Any thoughts on how to accomplish this or what might be available?

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