@heme725The passive detection range is 30 meters. You can see distance to objects when you're in drone, so you can know precisely where it will and won't detect. If you wan't passive detection, don't leave it higher than 20m.
If you leave the drone as detector for flankers, then best way is to leave it so that it covers as much area with the 30m detection radius, as possible (doesn't matter if drone is in plain sight) - enemies will either be detected or will shoot it - either way you'll know they're there. Works best for choke points (e.g., tunnels), since the detection will cover the whole passage.
I've seen a lot of crypto's who are afraid to lose their drone. If losing the drone gives useful information - just do it.
Passive marking is still not the most useful thing of the drone. If you want to detect enemies, it's much better to actually look yourself and stay far away from them. Get high enough and you can spot enemies across half the map.
As for EMP - who cares if enemy can hear the drone if they can't actually see it (behind cover) - they still can't shoot it. Of course using it completely in the open is harder, but you really rarely need help of ultimates to kill ppl completely exposed in the open...