I used to run a Star Frontiers campaign.  We got into Star Frontiers late - after Zebulon's Guide to Frontier Space came out.  Of course, at the time I didn't know that TSR was stopping production of Star Frontiers.  

I thought that Zebulon's Guide SFAC3 was the last item published for Star Frontiers.  It contained all sorts of rules & campaign expansions that really added a lot of depth to the game.  The guide also contains numerous references to Zebulon's Guide 2 which was supposed to contain further expansions - like Powered Assault Armor, Cybernetics, Bionics and more.

I waited a long damn time for Zebulon's 2 and it never arrived. . .    Anyway, I never noticed SF5 Flight of the Karoo, but knowing what I now do about TSR, I suspect that they had it earmarked with a product code (possibly even had a draft in production) when they decided to stop producing the game.

I know it is not a definitive answer, but I suspect that SF5 is kind of like SFAC4 (which would have been Zebulon's 2).   Anticipated, but never published. . .