Electronics - what are the ways to make sure that the distance between the tag and the sensor is less than, say, 5 cm?

Question

Electronics

There are two electronic devices, let's call them a sensor and a label, you need one of them to know that the second one is at a distance of, say, no more than 5 cm. The mass and dimensions are preferably minimal, as is the cost. It would be ideal if the tag were passive, i.e., would not require batteries / power, like, for example, contactless cards.
EDIT: Laser rangefinders and other optics are not allowed, because there may be, for example, a sheet of cardboard between the sensor and the tag. But not a sheet of foil.
Distance accuracy requirements are not important, if it works between 5 and 10 cm, then good.
But it is very desirable that both the sensor and the label fit into the dimensions, say, 4x1x1 cm.

Answer 1

Look for a mine detector diagram from a Soviet magazine for radio destroyers. The label will then be just a bolt.

Answer 2

RFID tags and reader + optional ultrasonic or laser range finder. Because RFID will not guarantee exactly 5 cm.

Answer 3

If you need analog and passive, then you need to use the anti-theft principle: the transmitter and tag have transmitting and receiving antennas tuned to different frequencies, but the tag sees the transmitter, and the transmitter sees the tag. The transmitter constantly transmits a frequency1 > The tag, when approaching a certain distance, receives a Frequency1 > The tag is powered by a radio signal and accumulates a charge > The tag transmits at a frequency2 > The transmitter receives a Frequency2. Frequencies must be high enough to only work over short distances.