Yes, they need to maintain active sensor lock to fire.
About EM detection, what do you consider "quite decent"? It's usually not easy to detect EM radiation from sensors.
IIRC active sensors emit radiation equal to their strength, meaning that Precursor ship with, for example, 280 str sensor, will be detected with 140 str EM detector in range of 39.2 millions of kilometers.
Again, I may be wrong on that.