Easiest way to do it would be to add a huge collision sphere to the player set to overlap all, add a begin overlap event for it, and check if overlapped actor has a tag like “metal” which you would assign to any detectable actors. If it finds something. add it to an array variable, and check the distance periodically (either in tick or a timer event and play sounds/display any ui info based on distance to actor.
Check out my posts and example project in this topic about making a geiger counter. Could repurpose code and change the tags a bit to turn it into a metal detector.