IMHO.. It's quite likely the normal shedding of the plants oldest leaf as Rosie says. I personally would not worry about it unless other leaves start doing the same. The only thing I would do if it were my plant is add some medium to fill the pot so as to cover the exposed roots and fill up the 'hole' you have. You might have to lift the plant as you don't want the bottom leaves covered in medium. Otherwise looks Ok to me. Hopefullly other people on this board will also answer.