First of all, it's kind of gamey and immersion breaking (I'm in the middle of nowhere and haven't seen a soul in days, and have flown around this system without seeing anything, but if I salvage something, a pirate fleet shows up without fail? Where did they come from?).
Well, just like how every fleet that is spawned in the game currently is out of approximation. Simply do calculation of distance from other colonies or bases. By using the salvaging skill, you can simply have a snippet of text by saying something in the lines of "By setting up a proper salvaging operation, the operation will emit lots of signals that any sensor in the system can pick up, thus altering whoever is running them."
Also, there are implementation issues, for instance, should the pirates scale with your fleet so that the combat is relevant? If so, then you will have to have massive pirate armadas constantly showing up out of nowhere in late game. If no, then you just add another chore to salvaging.
That's all in the spawn algorithm. If player is in habited system with lots of fleet presence, then don't spawn fleets that are enemies to who is already there. The whole process might most of the time not even spawn any enemies and that is fine.
Also, when you kill the fleet that shows up and salvage it, does that spawn another fleet?
Again, in the algorithm. If a fleet has spawned recently then simply don't spawn another one.
Also, I don't mind explicit salvage traps, like where you salvage and then get a message that it was a trap and a pirate fleet is inbound, but that can easily be done with existing mechanics IMO. No need to make me sit for 5 seconds doing nothing every time I salvage something.
The whole thing is sitting there for 5 or more seconds builds tension in that any moment, a fleet might appear to attack the player. The attacking fleet size could be equal, or even greater, depending on who is sending the fleet.
Say, the player is salvaging the remains of a huge battle in the system right next to Jangala while hostile to the Hegemony? Big kill fleet comes after the player. This is a chance for extra loot followed by a hard battle, perfect gameplay mechanic.
Again, I feel that this might be a great addition to the trade/bounty/exploration way to play the game. I mean salvaging should fit the whole, sector in decline feel of the game right?