How do you communicate with a military drone operator over radio? This article (and other similar ones) states:


  In remarks at the White House on Thursday, US President Donald Trump said the drone had flown to within 1,000 metres of the USS Boxer and had ignored "multiple calls to stand down".


For airplanes, ICAO standardizes 121.5 MHz and 243.0 MHz (military) as common emergency frequencies that aircraft "should" monitor when possible. If approached by an unknown aircraft, this is the frequency I'll try to raise the crew on first.

How does this work for drones? Does the same standard apply? Is it reasonable that in this case the USS Boxer's crew could expect that the drone is monitoring 121.5 (or 243.0) and that its operators will be listening?
Some UAVs carry VHF or UHF airband radios to communicate with air traffic control and other aircraft. The voice channel is relayed to and from the remote pilot operator via satellite or another form of datalink. I've heard UAVs taking with Joshua Approach in the Mojave Desert, and one time Joshua called one out to me as traffic and vice versa. I agreed to maintain visual separation, the remote pilot acknowledged and overall it was very similar to dealing with any other aircraft.

I have no idea whether they monitor 121.5 as well as whatever enroute frequency they're using, but it seems plausible that they could.