If you're talking pirate massacre missions, then I've found that non-wing ones spawn USSs with 7-8 targets in, while wing ones when you go to the nav beacon spawn give you a nav beacon scan failure message that says "you'll have to hunt them the old fashioned way", and generate mission signal sources like the OP described.
The correct way to do pirate massacre missions is to only take them when you see a bunch of them for every faction on the board all going to the same target system, because then every kill counts against each mission you have for a different faction.
For extra bonus points, stack a whole bunch of pirate lord assassinations too, because then the pirates will come to and interdict you instead of you having to hunt them down in signal sources, and they count as both the assassination and as a kill for any and all pirate massacre missions you might have stacked. If they're wing assassinations, then you can easily identify them as their signal source is threat 6, and not only does the assassination target count as part of your massacres, so do all the escorts.
I can't think of any possible reason why I would take a pirate massacre mission and not stack it with as many other missions as I can that also count against it. If you do it right or get lucky with the board, it's like free money for doing something you were gonna do anyway.