As far as I can tell the active sensors are on for the sensor vessel. Here:
https://www.dropbox.com/s/348ctt8dhbeypb6/screenshot5.PNGAlso, take a look at this screenshot again:
https://www.dropbox.com/s/a58h5u97u9euf8g/screenshot1.PNGIn my experience, the sensor rings for active sensors don't appear until they are turned on. So I am pretty sure they are on based on those two things.
Incidentally, the hostiles fired missiles at my ships and I am able to target the missiles and fire AMMs at the hostile missiles immediately.
Edit: Also of interest, the enemy ships are now generating active sensor emissions and I can select them as destination when giving the TG orders but still cannot target them.
Edit 2: I have a theory. The FCS that these ships have, has the following stats:
Active Sensor Strength: 168 Sensitivity Modifier: 110%
Sensor Size: 8 HS Sensor HTK: 1
Resolution: 95 Maximum Range vs 4750 ton object (or larger): 540,360,000 km
Range vs 1000 ton object: 23,949,474 km
Range vs 250 ton object: 1,496,840 km
Chance of destruction by electronic damage: 50%
Cost: 252 Crew: 16
Materials Required: 0x Duranium 252x Uridium
I'm thinking the issue is that the targets are indeed 1000 tons and I'm about 40m KM away from them. It seems like I'll need to close to within 23.94m KM before I'll be able to use this FCS to get a lock on the target. Is that the case?