Limber Holes
Your boat is probably sitting in it's cradle or on stands in a slightly different attitude than in the water. It is not unusual for most boats to have some water trapped behind structural ribs that will not drain to the bilge sump due to inadequate or non-existant limber holes. Besides doing all you can to open or add those that are accessible, not much short of major surgery will get the rest. If you drill out or add holes, be sure to thouroughly seal them afterward to prevent water from entering structural wood or voids.Adequate ventilation of the entire interior and use of dessicants will do a lot of good in helping to minimize this problem.