Mabey you have some tyvek going the wrong way as Mudmixer suggested. I would check around all your windows with the hose first and spray everything. You need to check from the top on down, then you will be sure. If it happens again this winter, it will be to late to fix it in the cold again.
Sorry, just telling you what I would do if a customer had this issue, it is a lot of investigation and work.