Are you sure?

As far as I know they use satellite links. So I imagine they can avoid sending signals towards earth. I also assume they use frequency hopping in a wide range so it's harder to track.

This random webpage claims that the (probably "a") satellite uplink frequency is "13.75 - 14.5 GHz". So you would have to dig up a downconverter that works in that band. Perhaps if the drone was overhead there might be enough signal that makes it to the ground to detect. If this became popular it might turn out to be worth the bother to equip drones with receivers to detect the local oscillator leakage from such early warning systems.