Seems I'm always on the run when I'd rather be savoring this new-found forum, lol. Before I skip on home, though, a huge thanks to all of you who started and have maintained this project--a phenomenal accomplishment. The level and detail of research is not like any I've seen on this subject (or many others for that matter). I appreciate very much your responses, and take each one very seriously. I know many of you have been doing this a very long time, and I'm grateful for your patience.
Getting to the possible US motive(s) for intelligence-gathering in the Japanese-administered South Sea Mandate islands, I like Malcolm Muir, Jr.'s observation as a starting point:
In December 1936, when the Japanese government announced its withdrawal from the treaty structure which had constrained the world's major navies for over a decade, the Roosevelt administration faced the necessity for renewed battleship construction. Unhappily, American intelligence on the Japanese naval rearmament was virtually nonexistent. Here was a matter of the utmost gravity, because the basic dimensions of any new American capital ship program (i.e., the numbers and characteristics of the individual ships) should be tailored to counter the Japanese challenge, especially since the long-standing American contingency plans for a war with Japan--codenamed ORANGE--called for the United States to advance into the Western pacific and force a showdown with the Imperial fleet....Imperial officers, weighing the battleship equation in the 1930s, counted on an impenetrable curtain of secrecy to build the ships that would trump American quantity with Japanese quality. (
Rearming in a Vacuum: United States Navy Intelligence and the Japanese Capital Ship Threat, 1936-1945])
While quietly proceeding with rapid advances in the size and speed of it's ships, Japan publicly crowed about its "spirit of nonmenace and nonaggression." Radio traffic analysis provided the US plenty of frightening hints as to Japan's new program, but didn't offer Roosevelt enough proof to justify the colossal expenditures required for the US to match Japan's escalation.
So why Truk...I should add again that, while Truk looms large, I don't presume that Truk could have been the only target of interest. I'm respectful of the distances you mentioned between AE's publicly-announced destination, and Truk. Japan was certainly fortifying islands in addition to Truk.
Nonetheless, Truk is very interesting for many reasons, not the least of which is that in the event of war with Japan, the US "ORANGE" plan, as it existed in July of 1937, called for the immediate seizure of Truk and Eniwetok:
From Truk, the US advance could go in any one of five directions. (Naval Institute
Historical Atlas of the US Navy, Symonds/Clipson)
After Japan's July 7th attack on Chinese forces, and Germany's annexation of Austria 8 months later, the US switched to a two-front plan with debate as to which parts of the "ORANGE" plan remained intact.
I include all that mainly to show how incredibly important Truk was in 1937--not just as a place to spy on Japan's naval activity, but one which required as much reconnaissance as possible in advance of potential US invasion. I also think it's incredibly ironic that only days after AE went down, the very Navy war plan which may have encouraged an overflight of Truk, took its first of two steps toward the potential scrap heap.
None of this means she flew over Truk. But it seems safe to suggest that in 1937, no one had more motive to collect data about Truk than Roosevelt and the US Navy.