Moogles are multi-faceted beings!
From what I can gather, they can (and do) hide at will, and can choose to make themselves known. The main-character (so you as you go through the main scenerio quests) is special and can see moogles in hiding who aren't trying to -really- hide. Padjals can usually see them as well, as prior to Heavensward, the only moogles we knew of were post moogles and the ones living in the Black Shroud.
As such, the moogles who deliver our mail could just be a game mechanic that's cute, but there are a bunch of postmoogle quests that go into more detail on their lives. It would appear that some have taken the task to stay out in the open to help out.
From what I can gather, they can (and do) hide at will, and can choose to make themselves known. The main-character (so you as you go through the main scenerio quests) is special and can see moogles in hiding who aren't trying to -really- hide. Padjals can usually see them as well, as prior to Heavensward, the only moogles we knew of were post moogles and the ones living in the Black Shroud.
As such, the moogles who deliver our mail could just be a game mechanic that's cute, but there are a bunch of postmoogle quests that go into more detail on their lives. It would appear that some have taken the task to stay out in the open to help out.