First of all, drones don't show enemy infantry/static units on the map, even when you're zooming in very carefully on them, and there's no option to do so.
I think drones should mark enemy infantry/static units depending on their range, bigger ranges would require you to zoom in more carefully.
An alternative option is to use the "Reveal target" option(default button is 'T') to mark those targets.
Then, their updated position would appear as long as the drone has a direct line of sight to the target, and it can be seen with the drone's thermal. (And the range is not too far)

Secondly, drones show their targets on the map, at any difficulty.
The reasoning for this is that in 2035(and even today)most drones would be equipped with net-centric capabilities and be able to obtain GPS coords of their targets/laser targets, and share them to friendly forces.