A small, blacked out SR FA off Cobra is damn near impossible to find.
Not if they never lost sight of you.
Right now there's no room to outplay an NPC of any rank and disappear, even momentarily, when in combat. They just know where you are at all times in a way that a real person can't.
They turn away when you get 8km from them, and that's about the same distance you completely leave sensors.
A real person looking for you will absolutely continue to follow your unresolved contact, even if they lose visual momentarily. The odds of you escaping anyone with any experience at a range less than that which NPCs would is essentially zero.
The stealth system is far from perfect, but the situations you and other describe as being impossible, impractical, or even unlikely really are not.
He's a master though. He's done this hundreds, if not thousands of times.
For the rest of us who aren't full of bull sheet
My statement was neither bull nor hyperbole.
Stealth combat has been a thing for a very long time and I've participated in a lot of combat with, and against, stealth vessels. Indeed there was a 9-12 month period, which coincided with when I was most active in PvP where it was hard to find a fight that didn't feature a stealth FAS or three.
Anyway, after my first serious encounter fighting stealth vessels (which I made a thread about in October 2014:
https://forums.frontier.co.uk/showthread.php/52142-Silent-running-combat-ship-fad) I've generally not found them difficult to fight, except when multiple ships that look identical are in the same hostile wing, which makes trying to focus one of them down a kind of cup game. If you think you, or anyone else for that matter, can catch my CMDR off guard, or shake him, using stealth, they are free to try. I essentially never leave Open and can give you a list of my likely haunts and play times.
An NPC who has reached those upper ranks should be assumed to have experience using and countering silent running and should be extremely difficult to shake.