Proximity Testing (Keyless Ignition System)

I’m interested in building a low power proximity sensor. I want it to be able to detect proximity in 3 dimensions similar to the say a Keyless Ignition System works in a car. I am not sure where to start. Does anyone know:

  1. What technology is used in cars?

  2. What other options I might use?

  3. What components I should buy to start experimenting?