Damp or condensation ?

Last week in the 'van we had 2 days of heavy rain and high winds.when my wife emptied the wardrobe,end bathroom with wardrobe in corner,there was a strange line of damp.It ran for about a foot along the top of the wardrobe shelf and a couple of inches high.Taking out the drawers below there was no damp and nothing at all above the damp area.The patch was on the outside wall.

The panel is really solid with no sponginess.Could this be condensation as i put a damp jacket in there or is it the dreaded damp.It just seems strange that it never went below the shelf.Any ideas before i have to open my wallet !