I've been reading a lot of this speculation for some time and I agree that it is easy to think of things that a castaway should have done to make sure a search plane (if they could have guessed there were planes) or a searching ship would have known they were present. However, it seems to be that these speculations are founded on an assumption about the possible castaways that is itself a speculation.
Specifically, what should we assume about the physical, emotional and mental state of a castaway in AE's situation (if, in our theory, we assume she was present). The Niku theory assumes that after an exhausting, lengthy flight in an extremely noisy cabin at high altitude, AE and FN went through the incredibly scary emotional ordeal of being lost in the middle of the Pacific as fuel ran low. They then (according to this theory) made a "hail Mary" effort to find an Island and happened on Gardner. They may, or may not, have known what that island was (it didn't match the outline in the charts - if they had them.)
They attempted (according to our theory) an incredibly dangerous landing on a reef at low tide. We have no way to know how that landing went. Was it an incredibly rough landing (balloon tires on sharp coral?) Were they injured? Was FN still functioning after the landing?
Then our theory assumes that they made at least one and most likely multiple trips between the lagoon and the reef. We know that the surf - especially on the outside of the reef - was extremely difficult. The trained and well equipped boat crews that rescued the Norwich City survivors had a very difficult time getting ashore and speculated, at one point, that it would be impossible to do so. Some of the Norwich crew died getting ashore (in a storm) but all accounts mention a great number of sharks. Admittedly conditions inside the reef should be far better but seriously folks, we are not talking about Mike Phelps here. We are talking about exhausted, possibly injured, people swimming to and from a wreck that is partially submerged at high tide in an area frequented by sharks.
Now add what their experiences much have been ashore (again, assuming the theory is correct). No or very little available water (one of the other parties that explored the island resorted to drinking water that had been puddled in a guano deposit). Resting - at night or during the day - must have been hellishly difficult because of the predatory crabs. If FN was still functional then perhaps they could have taken turns but even that would have been difficult for exhausted, possibly injured, people. They could try to start a fire to protect against crabs but that, in itself, might not be easy for exhausted, possibly injured, people.
So put that all together and what kind of speculative assumptions should you make? A physically, emotionally and mentally fit AE would have likely left some visible signal. Would an exhausted, emotionally wrung out, possibly injured and potentially panicked AE have done so? Could she even have fallen asleep in the shade of a tree following an unsuccessful search for water?
I think the answer is we don't know. Depending on the assumptions you make, the lack of a signal is very strange or perfectly understandable. Either way, I don't think you can treat the ABSENCE of a signal as proof against the hypothesis. It is simply one more factor to be weighed in the balance with ointment jars, lost skeletal remains and sextant boxes.
Anyway, my two cents. Now I am off to set my DVR to record Sunday night's show.