to expand how we normally detect drones is you use a micro-doppler radar which tells you when something is moving too fast but excludes bird wings so no false positives. this is always combined with rf detection because again 'normally' someone is using rf to control a drone. neither of these work against these drones. when you spot a drone you track it with thermal because it has engines giving off heat but this can be thwarted with clever engineering/countermeasures. when you get close enough to a drone you use active ir systems but that can be defeated literally using glass encasement or military methods we don't know about i'm just an optimal control lacky that programs approach and automated landing.  tl;dr if you can't see it (night, no lights) and thermal/active ir doesn't work then you can't follow these drones unless you have audio sensors.
Canada