Damp patch after heavy wind and rain?

This has hapened 3 times in the last year. After really heavy rain and high wind a damp patch has appeared on the upstairs landing below and to the left of a window. Had a builder make repairs to the external render and resealed the window during the summer. However last nights storm brought the same problem back, but in a slightly different position. Render looks ok and we repainted a coupleof months ago. Any ideas?