It seems there may be more than 1 way to unlock a sector.

We were still rank 22 and hadn't unlocked sector 8 yet (was using invincible forecastle along with enterprise aftcastle, so 76 surveillance). I had also been flying 6-5-7 to increase time/range efficiency. On this trip we took a route that would take us closest to sector 8 (6-7-5). I wondered if the trip from 6-7 would do the trick.

When she came back, our flight log showed that we had actually unlocked... sector 10! Which happened going from 7 to 5.

But then when I finalized the report it said that we had unlocked sector 8! It happened at the same time that it told us we ranked up to 24. I'm not sure why it would show sector 10 being discovered in the flight log and not show sector 8, except maybe it was unlocked because of a different condition.

So she is on her way to 8 now, time to get that 2nd airship!