Couple a weeks ago, we were running FCoB and our NIN DCd, about 10-15 mins later his char is still in world with the DC symbol and he is unable to log back in.

Uncontrollable things can and do happen. I've had it where my connection to the internet has suddenly dropped off (couldnt resolve DNS blah blah) during dungeon runs. I bet I aint the only one that has happened to either.
What I'm getting to is that there is no way to tell the difference between a forced DC and an actual DC.