Are there any products that can act as a locator beacon, in perhaps a pager form factor? They listen for a signal from a powerful base station at frequencies low enough to deeply penetrate RF opaque material, and upon command, could key up and transmit at max power while using Time of Flight to triangulate (from a network of base stations)? 900mhz comes to mind, but perhaps there are alternate, more optimal frequency bands for this.

It's a local version of https://www.findmespot.com/