To explain what I mean about sensor capabilities I include a random selection of sensors I put on a sample ship
Aru Sensor Systems Active Search Sensor AS199-R100 (1) GPS 32400 Range 200m km Resolution 100
This is a long range active sensor similar to yours, at long range it will detect a 5000 ton ship at 200 million km, however it will only spot a 500 ton ship at 8 m km and a 50 ton contact (like ground forces) at 500k km. So your ship at 100m km will only detect 5000 ton or larger ships. This occures as soon as the ship is in range waiting makes no difference , you will never see smaller contacts unless they come closer
EM Sensor EM15-270 (1) Sensitivity 270 Detect Sig Strength 1000: 129.9m km
Thermal Sensor TH15-270 (1) Sensitivity 270 Detect Sig Strength 1000: 129.9m km
These two passive sensors are similar they will detect a signature of 1000 at very long range, they will detect a signature of 100 at about 40m km and 10 at less than that. Again time makes no difference either you have a contact or not.
For reference Earth in my current game has a thermal signature of about 50,000 and EM about 150,000. The automated mining outpost on the moon has both signatures of around 500, a simple deep space tracking outpost has around 10
ELINT Module (15) Sensitivity 165 Detect Sig Strength 1000: 101.6m km
These work like the EM sensor for detection, but slowly if they spend a long time detecting a colony you will get occassional intelligence information(random information about colonies, systems or technology) These ELINT systems are relativly huge each of these 15 modules is 500 tons. I almost never use ELINT so I can't help with any information on how fast they pick up things or exactly what the limits on what they find are
Hope thats helpful