If you do a search on ScubaBoard, you will find a number of references to this issue. On some Subal models the leak detector is quite sensitive, and even a small bit of moisture will set it off. It is doubtful your rig is perfectly dry: even a small amount of condensation can set it off.
Try taking the battery out and using a hair dryer to dry the contact points. You might even bend the contacts up a bit from the housing bottom.
I had an issue with my D70s housing where the back plate was not properly aligned and a very small amount of water would enter the housing. The D300 housing has been redone, and it has really good.
Good Luck,
Dan
