I hate to say this, but if you have looked in the sump,overflows, tubings, hoses, live rock, sand, all around the tank, stand, under all the pieces of furnitute in the room, looked for holes in the floor & walls, any nearby closets, baskets, and you still have had no luck finding it. That only leaves your room mates. It is not all that hard to catch an eel in a tank. I pray this is not what happened, and it is still in the tank hiding inside a rock.
When ever any of my eels escaped I have always found them by their tank, overflow box, or in the sump, except of course for the poor ribbon that got sucked into my Iwaki pump.