I vote for Ralph as the winner!
It can be one of a number of things. You may get to a point where it is just there, nothing more you can say about it.
The best tip is to keep the area as clean as possible so the water doesn't get nasty. A dry bilge is nice, but there are millions of ways to get water in there so there isn't a whole lot you can do about it unless you take extreme measures. It is a boat, it is in water, water will get in it. I say make sure the bilge pump works and keep on sailing!