There is a lot of talk about missing apprentices both in the game and elsewhere.

http://www.uesp.net/wiki/Skyrim:Ilas-Tei

Is this a real quest? I found the mentioned missing apprentice mages in various locations, now what? Is this like the missing Goblin Wars quest in Oblivion where everything is already in the game except for it to be active?

Good question. I've actually (on my "explorer" Khajiit) found a BUNCH of people said to be missing apprentices (when you look them up in the wiki). But I've yet to get a quest of any sort on the toons who are College members.

I think it's just fluff added in, adds a nice little twist, but not much needing a quest for.

Dialogue for the quest exists, and it can be forced to update through the console. You immediately can report the death of any apprentice, even if you don't have their unique item. Gestor will mention rewarding you (just for finding, or not finding, as the case may be), but gives you nothing.