So These devices should be 2-way, App on your phone to buzz your keys if those are missing in action, and then obviously keyring to set the phone off buzzing. I would say bluetooth tech would work for this but distance is limiting, although if you were to walk around the house with this thing transmitting, as soon as your device came into range, it would start to buzz at you
as an engineer in a similar space I realized the fatal problem with this type of product... Each device must have a very low battery drain always on receiver. I suspect the many poor product reviews I read was due to dead batteries, just when you need to use the product. After thinking about this from an RF and battery drain perspective I don't think there';s a $25 solution. The cellphone solution that transmits a Geo location every 30 minutes is the best compromise. Have lots of key copies and especially a key in a magnetic box under the car~!!!! the old standby solution.