I hope you're proud of yourselves. . . 

So between rolling over some precursors in 5. 6, having pretty decent tech in my new 6. 21 game, and this series of posts, I felt pretty comfortable sending a cruiser squadron through what I thought were empty systems to double check them before I sent in the geo-survey ships (my grav-survey ships have cloaking, thermal reduction, and huge passive arrays).  I light off my huge active sensor, and who should come out to greet me but a whole swarm of precursors pulling 13317 km/s.  Including ones up to 34,600 tons.  I've been bushwhacked before by precursors, but this was just silly.  200,000+ tons bearing down on me at more than 3x my fleet's top speed.  Mind you, the missiles they fired sucked badly.  Like 98% hit chance with my AMMs badly.  Out of a few massive waves only 3 got through, and only then because I brilliantly hit the 5 min increment instead of the 5 sec one when I was trying to set up my last ditch fire.  The 3 that did sneak past had bloody 29 point warheads though.  Needless to say I got run down and obliterated by lasers well short of the JP. 

I think your intelligence agency makes their estimates on sensor range using your tech level, until proven otherwise through salvage or capture

If I can see any of them at all then surely I should be able to see their largest one, perhaps they had an overly gigantic one and I was too zoomed in to see it.

That's happened to me before, in exactly this same situation.  Feeling smug and confident and deciding to zoom out to check up on some other things... total mood turn around...

They seem definitely toned down in 6.1 for me as well.
Much less dangerous than before.

Update on the precursors: it looks like they are not obeying radar rules, or are targeting missiles with EM emissions and not active locks. I'm outside their range and getting missiles shot at me. I'd turn off my largest scanner if that wouldn't also turn off my anti-missile sensors to see, but I can shoot these down as well(ran into 4 ships that could put out a lot more though, big mistake getting into their engagement range).

Same thing as TheDeadlyShoe, your EM sensors might not detect all their Actives. Also, the range that is specified on the map is only an estimate and is purposefully smudged - it is also based on _your_ tech level (your EM and Active levels).

Update on the precursors: it looks like they are not obeying radar rules, or are targeting missiles with EM emissions and not active locks. I'm outside their range and getting missiles shot at me. I'd turn off my largest scanner if that wouldn't also turn off my anti-missile sensors to see, but I can shoot these down as well(ran into 4 ships that could put out a lot more though, big mistake getting into their engagement range).

I spotted 5 9100 ton ships headed towards my taskforce of warships, with my geo survey ships. the geo survey ships were able to run for it, even after being in their sensor envelop, which was surprising but perhaps they wer focused on the bigger threat. The missiles that they fired at my ships all had speeds that varied by a few km/s, which made their salvos spread apart, to the point where my 10 15cm V3/C3 railguns were more than adequate to shoot them all down. likewise the missiles were much much slower than 5.6 precursor missiles, and the ships themselves were about 2000 km/s slower as well. Finally, the missiles must have been em targeted, I don't think that they had an active sensor that could spot my ships, other than the shorter range sensor that spotted the geo ships.

I saw no atmosphere streaming so these were likely new precursor ships.