Of course, you wouldn't know this if, like me, your characters as of Fallen Empire had already progressed up to that point and had no reason to turn Exploration missions on in the first place. Characters who still needed to Level, of course, had far greater precedent to encounter this.
I myself had only just allowed Calph to access her Exploration Missions for the first time a couple of weeks ago - the exact reason for doing so eludes me - and had found myself baffled by the amount of missions which she apparently hadn't yet done. Sheriff Encot's hostage rescue is a mission I've always done, and yet she hadn't done it?
Eventually I realised why this was so, but it's still something which caught me off-guard.
Funnily enough, I had actually been looking for an excuse to go back to certain planets and finish off the missions which I had never done - either on Calph or on any of my characters - so this seems like a good time to start doing so.
By the time this game is done, I would love to turn around and say that I have completed every single mission which the game had to offer. I really do like a lot of the Exploration Missions, so I wouldn't mind completing them again, and I know that Voss and Corellia have literally dozens between them which I have never ever done simply because most of my characters hit 50 on or before Voss itself.
This re-questing has also had the nice side-effect of allowing me to catch changes which I wouldn't have thought to be a logical change. Most notably, the overarching mission for the Tatooine Bonus Series actually has similar rewards to a Heroic Mission nowadays.