Originally Posted by
Anonymoose
We have only a few references to it and all are suspect. 1.0 had a line about how it lets them see in all directions at once, but that was an unconfirmed Eorzean suspicion from a guy who said they implant magitek into their bodies. Nero could have been using his to out the spy early in 2.0, but he said "frumentarium sees all", which implies that the Garlean intelligence agency just told him who to kill (The Frumentarii were the wheat collectors of ancient Rome, and also their secret service).
The only legitimate reference to it that I remember is when you Echo-blast Cid's amnesia and he says, "That light─it was you, wasn't it? It's surprising how few people know this, but all pureblood Garleans have a third eye. Perhaps mine helped me recognize you─or perhaps it was just a lucky guess."
On one hand, you did just Echo-blast his amnesia away, but on the other, perhaps that's why he saw the Echo as a light. Not many people that you resonated with to see their past actually knew you were doing it - Raya-O-Senna was one of the few.