In my current save (5.1HF1), I am ocassionally snatching SCA destroyers. And it almost looks like friendly factions are helping me.
Once I start boarding an uncovered ship, and it doesn't attack a station or a friendly non-police ship by chance, sector police seems to attack, but not destroy. I've boarded multiple SCA destroyers now, the last two with scanning twice for getting crew info. I've avoided blowing their cover before, since I thought they might get knackered by police while my marines are still boarding it.
During boarding, both SCA Phoenixes got attacked by sector police (ANT in Argon Prime, MIN in Grand Exchange, I own all the police licences), and I thought they were done for. Police kept firing and firing until the Phoenixes shields were down and then ... stopped. Only to repeat the shield drain at various stages. At least it looked like that.
It might be, that all this police was equipped with just shield drainers, alas I lack the rep to peek in on that remotely. But that'd be quite a stroke of luck.
In fact, no one seemed to care much in the 15 to 20 minutes each fight went on until boarding succeeded. MIN with its massive police presence in Grand Exchange completely ignored the one-on-one between the S and the Phoenix, only a MIN Phoenix patrol destroyer turned its head and started following without intervening. Probably to cheer its bored crew up (there it is, the secret to raising morale).
In Argon Prime the ANT S fighter put on its nifty crown, and had the impressive force of another single S fighter join in, which was all they had around, since the rest is busy getting killed by Xenon.
So, my question is: Once you start boarding an uncovered ship, and it doesn't attack a station or a friendly non-police ship by chance, sector police will attack, but not destroy, even if their factions are enemies ?
This sounds pretty unlikely to me, but then, this is X4 ...
Note, that I am not firing on my targets, so they still are able to move and do stupid things. Have seen some rather quick repairs, so I stopped trying. Careful approach, keep a close eye on target speed and heading, let it run into my pods dropped as close as possible, preferably in a spot where the target has a hard time shooting them down on their few hundred meters of approach.