I believe I got the ASS nailed down but what about the passive sensors?
Think of your sensors as active and passive sonar on a sub.
Active sensors are your active sonar
EM-sensors see the EM emission from other ships, which is created by active sensors or shields
Thermal-sensors see heat radiation, created from engines
Yes, having your active sensor on will let you see your enemy out to 50 mkm, but if your enemy has good passives, he will see the EM-signatur of your active sensors from several hundred mkm.
All of my own ships have small Thermal and EM sensors, but those are just backup. The real eyes are on a fleet-scout or two, which will accompany any battlegroup (they also mount much much larger actives. As a rule of thumb: 1000t Res-120 active, 1000t Res-16 active, 500t Res-1 active, 750t EM passive, 750t thermal passive. As sensor tech advances, the Res-1 and Res-16 actives might get a bit smaller)