By 'fog' I meant that unless a ship is within scanner range of something, you should nor see anything, bases, ships, colonies, etc. Of course once you found a base or colony then they would always be marked since they are hard to move Of course they would stay marked even if someone destroyed them unless you scanned them again (or exchanged maps with someone who had more updated information).
Is seems like you should not know a fleet is anywhere near (or even in) your system unless you have ship/planets/bases with the approporate scanner. Much less know that ships are headed your way from a completely different star system lightyears away. Aurora handles scanner systems the best of any game I have seen. Of course I do think having to worry about how sentistive versus the range of a sensor is a little excessive. Realistic, yes. Adds a lot to the game, not so much.
Imho, there should be 4 different types of military scanner tech that supports two componats for ships and two componats for bases (and very large ships). The first tech pair would cover scanning hyper drive emissions. One tech would increse range, the other sensitivity. Where sensitivity increase would allow ship to be detected sooner based on size. So the 'more' sensitive your scanner, you can start picking up smaller ships based on the range tech, while larger ships could be detected even further away. The other pair of tech would cover non-hyper scanneing and follow the same pattern as the hyper version.
So without any scanners, it would be entirely possible for a fleet to just 'appear out of nowhere' versus how it seems to work now. Also, if you have the hyperdire scanners tech up but not the other, you could see them coming, but them they could disapear within your system once they got there . Unless they started blowing things up of course
The countermeasures tech already in the game (which I never research either ) could be used as a 'anti-scanner' to the above, so the higher your countermeasures are, the range/sensitivity of the scaner techs are reduced (but not eliminated) by some value. Of course, you also have counter measures to counter measures too. But I do not think adding in yet another componate to research would add anything since the 'ECCM' (Electronic Counter Counter Measure for those not of a military background ) could be assumed to built into the scanner tech itself.
Just some random thoughts as I hate having tech that seems to have no importance in the game. So if it not doing anything in game terms, why have in there at all?