It happens.

It's not that rare for Americans -- under official cover or not -- with US diplomatic or US official passports to travel in casual clothes; that indeed may include tropical/beach city-wear. It's just that many countries have a lot of people who think that the idea of having a diplomatic passport means having some kind of glamorous role for those who are dapper dressers and smooth talkers -- even though a few summers hanging out with the Foggy Bottom drinking crowd would soon divorce people of those stereotypes.

If a young male entry-level USFS employee were traveling by himself back again to his assignment in Bangkok using a diplomatic passport and was wearing a suit and tie on a late Friday or Sunday night, that would seem more odd to me at BKK passport control. But that is a consequence of a bit too much familiarity with what happens even when perhaps it shouldn't.