[Phantasm]

In my own setting, there are hundreds if not thousands of slow sleeper colony ships still plodding along at lightspeed or just barely over it (these were developed about 150-200 years before the setting's ~2500CE date, so are at least one TL behind the rest of the setting). So yeah, there's a possibility of plenty of "lost colonies" in and beyond the frontier, some of them intentionally "lost", ala Pern. Many of them may still be en route to their intended destinations.

In many cases, in my setting, "the frontier" is "we've explored this area and opened it to colonization, but the exploration isn't complete". With any luck, you may be able to get help from the Colonial Defense Force on a 'timely' basis (that'd be like Fort Laramie sending out the cavalry; sometimes help arrives too late). "Going beyond the frontier" is possible, and perhaps even desirable for some folks, but help from any kind of authority is pretty much non-existent.