I apologize up front for this rant! After my post, "Contradictions Going on with Drone Regulation and Enforcement Are Amazing!" I decided to see if miniature transponders exist. Turns out they do and the video to the right explains how they could work on UAVs. I was pretty calm about this until I noticed the date on the video; 2012!! Model airplanes had transponders before drones became popular!
The other somewhat related article that I've recently noticed is that European airports are implementing DJI's GEO 2.0 geofencing technology in 32 countries.
It appears to me that the US FAA is not even engaged in any of this! I guess on the positive side, it means that here in the US, we can just do what we want with drones.
Update: Feb 23, 2019 - There's a new player in done manufacturing that adds new urgency to this subject. The Kalashnikov Group, the Russian company that gave the world the iconic AK-47 assault rifle has unveiled a suicide drone (KUB-UAV) that may similarly revolutionize war by making sophisticated drone warfare technology widely and cheaply available. It can fly for 30 minutes at a speed of 80 mph and carry six pounds of explosives. See this article on the WA Post for more details. At least they aren't available on Amazon yet. Just wait a few weeks.