We don't know what they were doing. We only saw what happened at Ceti Alpha 5.
Which is more or less
the point: it allows for all those interpretations where scanning for the absence of CA VI would be highly exceptional and unlikely.
Let me be clearer: Ceti Alpha 6 exploded six months after Khan got there. The Reliant didn't come until 14.5 years later. By that time, there would have been nothing left of Ceti Alpha 6 to even register. That's completely different from what happened in "The Doomsday Machine."
Ah, I see - a fair point. Although I don't really see a couple of decades making a difference in the real world: it wouldn't be enough for either the complete dissipation or complete reagglomeration of the planet, assuming it went the way of those "DDM" worlds which "broke up" and left a lot of rubble behind.
In any case, we get Starfleet's response to star systems suddenly missing planets. It's a mystery that must be studied at close range, but also literally nothing to write home about (neither Kirk nor Decker appears to have called for reinforcements until it was
way too late). The biggest difference here IMHO is that both Decker and Kirk have the spare time to study the phenomenon, while Terrell is explicitly busy with a mission.
Which is a bit surprising. Yes, the impression is that Kirk and Decker are in the desolate far frontier (possibly even close to the edge of the galaxy, to allow Spock to postulate that extragalactic origin for the DDM), thus probably with fairly generic assignments (because there's nobody they'd know or care about out there). But why are two starships following not just each other but their own tracks (Kirk has been to L-370 recently)? For idle loitering, that's particularly inefficient... But I digress again. Planets go missing, Starfleet investigates, and we find out about the limitations on doing so,
even when the spoor is fresh.
We know how fast warp 4 is because it took the NX-01 four days to get to Qo'nos at that speed. Which means that based on the Klingons' level of technology at the time, they should have reached and conquered Earth a long time ago if they were that close.
How is that a problem? At any level of technology, aggressor cultures can get to victim cultures in a reasonable length of time for conquest. Yet the universe remains largely unconquered. Generally speaking, we can always assume there to be obstacles to conquest, and it's left as an exercise to the audience to come up with the obstacle of the day. My bet here is Vulcans (the ones to make ill-informed first contact with Klingons "centuries" before that TNG episode, resulting in a brutal war where the technologically inferior Klingons got extra bumps on their foreheads).
Timo Saloniemi