I would top the oil up, tbh even if it was only 3/4 full there would still be enough oil in it to stop any real damage in most circumstances, obviously not ideal but not the end of the world. I have heard that in some cases gearbox's can run perfectly happily with only 1/4 full of oil, the nature of the movement moves the oil around enough to lubricate everything, the main issue I assume would be heat, or more precisely the inability to remove it, but i am just guessing!
If it had lost all its oil it would be very obvious, oil on drive etc...
My gearbox whines like a b**ch in 1st, 2nd and 3rd but has plenty of oil in it, I don't see it as a real problem and am not expecting it to give up any time soon, so I wouldn't worry too much.