Well, it's new in hiking clothing, but RECCO chips have been in ski clothing for quite some time. Most large ski areas now have handheld RECCO detectors, as do many SAR teams.

For some time teams have used hand held detectors from helicopters. This involves having an operator sit in the open door (tethered in of course), operating the unit. I think this was what was done on the 2018 Mendenhall Towers mission. More on that mission with a photo of the face is at American Alpine Club.

I think the RECCO detector on a helicopter long line is relatively new. Sounds like it has much better detection range. Until now the short range of existing handheld units has limited RECCO use for lost person (non avalanche) searches. This newer detector might make it more useful for wider area searches.

Rafowell posted a good link to the underlying technology. Basically anything with a diode in it can give a response (maybe weak) to a RECCO detector. Of course the RECCO chips in clothing are tuned to give the strongest possible response. I've heard of at least one avalanche recovery where the RECCO detected a buried snowmobile, presumably responding to components in the ignition system. The body of the victim was then found close by. When using hand held detectors the operator often has to remove any other electronic devices from their person, such as cell phones, watches, radios, etc. Having that kind of device in close proximity to the detector can sometimes give a spurious response.

I'm a bit surprised at the cost quoted for the RECCO devices to attach to helmets or packs. I think the chips themselves are quite cheap to produce. Then again, I suppose RECCO needs to make money to stay in business and pay for their development costs, such as the helicopter based system.