NPCs are basically there to farm stuff from - kernals, plasma, modules, or whatever. Making them too hard to find is irritating.
So I can see two ways of doing this that makes it fun instead of irritating.
1) Have NPCs roaming as suggested in Jita's OP. Rather than having them spawned by a scanning system, have a mini game of tracking them down using a signal detection module that works by the same principle as artifact scanning. Because they are moving you also have to take their movement speeds into effect. But you could also bump into them by accident. As for miners - currently NPCs all ignore you if you don't activate any modules or move, so what they need to do is switch off their modules and sit tight until the NPCs pass. Or even have decoys that they can drop on the ground like field containers, and only work on NPCs - put them a little way away from you so when a roaming NPC gets too near the miner activates the decoy and hope the mob gets pulled in that direction.
2) Have static NPCs, but roaming a much wider area, as well as static ones spawning to protect buildings, as suggested in Alexander's post. Again, this wouldn't interfere much in with miners, but it would make the mobs a bit more dynamic.
"...playing a game is the voluntary attempt to overcome unnecessary obstacles."
Bernard Suits, 1978