From the blog post referenced in the above link:
"My alternate hypothesis was that a Bushnell surveyor inadvertently left the box for the Brandis sextant somewhere in the vicinity of the castaway’s remains, where it was found a few months later in the search for the castaway’s personal effects. Much of Nikumaroro is covered in forest and scrub and so it is conceivable that a Bushnell surveyor could have passed close to the castaway’s remains without seeing them."
A couple of observations regarding this idea:
1. Gardner (Niku) was far less overgrown in 1938 than it is now. The idea that a Bushnell surveyor could have happened upon the sextant box without seeing the remains later collected by Gallagher seems a bit far-fetched. The 7-site was much more open in 1938 than it appears now - hence the reason our castaway chose it in the first place. In addition it is highly unlikely in my opinion that a single surveyor would be out and about on his own. Buddy-system applies in strange territory. At least two and perhaps more would have been present. And if you presume the individual handing the sextant was an officer, then perhaps several people would have been in the area. The idea that a partial skeleton lying at the base of a large tree goes un-observed is hard to fathom.
2. As I understand it the 7-site is not all that close to the Lagoon and if the edges/beach of lagoon is where they were working their survey lines, what reason would they have had to be so far afield. Perhaps simple curiosity, I guess. Maybe to get someplace with a breeze.
3. I've never handled a sextant but it seems to me that for a precision instrument to be carried about the island outside it's protective case/box seems very unlikely. I expect people (officers) trained to use a sextant are also trained to care for them properly --"put it back in the box when you're done, Mister Smith!!" Carrying a sextant in one hand while bush bashing across the island seems counter-intuitive. And if he did so and appeared back at the ship without the box, the CO or XO would have sent him back to get it. Alone. As punishment.

Anyone have any thought on these points?
I have to admit though the numbers on the documentation cited in the blog post are very interesting.