Talking to Dennis today it seems that you should empty the water container if you plan to leave the boat for any period of time. I only just started to use mine this year, and between the end of our early August holiday until last week (mid Sept) have mould forming in the forward locker. Dennis noticed the same issue when he left water in his.
Luckily for me there was nothing in the locker to be bothered by the mould, and I've managed to clean it all away. I won't be leaving water in there again. Dennis has made some changes to his already too.