I've asked this question before, but never really got a definitive answer. IIRC, it was something like 'There could be, but there shouldn't be'. As redundant as that may be, that's probably a no. Just too much territory to cover.
As I understand it, all systems are supposed to be within visible sectors. So there may be one outside of/between sectors, but if there is, it's basically an error.
So assume all hidden systems are within normal visible sectors.
____________
"May the Grace of Ara go with you, and His Vengeance be wrought upon your enemies."