I just noticed that my middle toenail is almost gone. there's a clear ultra paper thin layer left where it was.. it doesn't hurt. what could this be?

Possible: That not all of the nail came off, might be just a portion. Since there is no pain i would not worry but inspect daily to make sure no redness or pus is noted, if this happens seek medical attention.

Toenail trauma: It sound like you may have had some kind of trauma to that toenail. There is major trauma such as kicking a piece of furniture and tearing the nail and there is micro trauma such as the repetitive hitting of the toe against the end of the shoe. Both of these can cause the nail to come off. If no pain, redness, swelling or drainage, i would leave alone. The nail may start to grow back with time.

Possibly the nail: The thin paper layer may actually be the innermost layer of the nail and not completely avulsed. Leave it alone as your nail may grow out normally. Best of luck.