The spare tyre well on my 52 Primera keeps filing with water. Why?

Every couple of months I check the well in which my spare tyre in housed and find that it is filled with up to 3 to 4" of water. I can find no outward sign of a leak, the seals appear in order and there is no sign of any damp in the actual boot space above the well which would indicate where the water is coming from. This in turn makes the car almost impossible to clear of mist in the winter months. Anyone suffered a similar fate?, I am lost for ideas.