It is simple. Have you ever used an ordinary hand held magnetic compass?
If you hold it down by your waist and you have a heavy iron/steel belt buckle the the compass always points to or away from you, more due to proximity than mass of the metal to compass.
If you bury one center of parking spot how far away could a car or truck park and the sensor point to that? Next slot over? 2 slots over? Reality check: take a hand compass out in a parking lot and see what effect cars have on it at different distances. It tails off real quick.
That's using a digital compass which is passive. You can go active and use a Hall to monitor the field, even pick up induced fields. Have a coil circuit put out a strong burst and look for a return every 10 to 30 seconds.
You might check out capacitive sensing. People can be sensed standing a meter from a wall that was painted with conductive paint using capacitive sensing. You'd also do well to find a metal detector circuit.