Any wireless technology to find the distance between transmitter and receiver
Depending on the distances involved, sensing / timing precision available at master transceiver, response speed at slave transceiver, and computational power available on the master device, Time of Flight mechanisms may be usable for distance estimation.

ToF measurement involves transmitting an identifiable, unique bitstream  from the master transceiver, echoing it back either passively or actively from the slave transceiver, and measuring either the time taken for the round trip, or for shorter distances, phase differences between outgoing and incoming signals. 

A useful paper that details this approach is this one by Steven Lanzisera et al, UC Berkley, June 2006. 

It is noteworthy that ToF mechanisms are far more reliable than any signal strength measurement mechanism, because the impact of environmental conditions on the speed of light (or, to be precise, the speed in air of the RF frequency chosen) is marginal compared to the impact of environmental conditions on signal strength. 

A point of key significance is that this sensing mechanism is severely impacted by reflection paths for the radio signal, which result in multiple longer round-trip radio paths, and thus multiple invalid distance values for the one minimal-path value. In other words, such ranging mechanisms provide poor precision indoors, compared to outdoor.

Also, the presence of (in effect) a long conductive path that would work as a sympathetic antenna would reduce the perceived time-of-flight, thus generating a shorter detected distance than in reality. For instance, measuring ToF while both transceivers are close to a long metal pipeline would significantly invalidate results.

Can RF ToF ranging be done with typical DIY electronics? The paper referenced above shows that it is feasible. That is not to say that it is simple, or that it is computationally feasible using a low-cost microcontroller development board such as used by hobbyists.