>>96227797
Probably not very much. Basic bitch guard squads are equipped with vox units and auspex devices that can transmit clear signals across hundreds of miles - as well as the technology to jam such devices. The Imperium's technology when it comes to signals and waves is clearly far in excess of whatever we have. They have devices that can detect life signs in real time so I doubt it would be hard detecting drones from a long distance.
Seeing as how 'tiny drone' technology is already present in 40k there's obviously some reason they don't widely use it in battle. Their battlefields are awash with the kind of high-energy radiation that would play havoc with unshielded electronics (and guidance sensors). Every plasma shot or plasma-based explosive is spewing gamma rays out the ass, extremely high powered lasers flying everywhere, god knows what kind of waves come out of melta reactions which are some kind of nuclear fusion. And the aforementioned extremely powerful communications signals. Tau drones are probably about as small as they get get while still being sufficiently shielded.
The Imperium also has automated CIWS technology so widely available that they're used as semi-disposable sentry turrets, the Tarantulas.

>>96229674
Picrel