I think you've found an old bug, I'll quote my old post on the subject:
I never had the passive sensor circle working on Jupiters moons, I thought I had reported at a while ago.
I set up sensor platforms on a ring of asteroids around Uranus's as well as one on a small Uranian moon, every one showed a ring except for the Uranian one.
Just checking this, It seems the sensor is showed on the moon, but the circle is too large. With one tracking station and at "to detect signature strength 1000" Asbolus, Nessus, and Crantor show a blue circle 580 million kilometers in diameter, however on Caliban the sensor ring is 1000kilometers at Signature strength 6 . This is actually what I reported a while ago, I just forgot the exact details.
I think that has a typo, and I meant 1000 million kilometres at sensor strength 6.
Interestingly here's the first post for this "bug"
http://aurora2.pentarch.org/index.php?topic=5098.0Way back in 2012 it was discovered that the displayed sensor ranges were using the old range calculation rather than being merely additive, it was quickly fixed, however much later in that thread it was discovered that on moons the range was still incorrect.