I just now cam across your message on this web site. You may have the same problem we have in our condo building. With our windows, it appears the window manufacurer decided only the west facing windows needed to have caulking on the inside corners at the bottom of the window frame. Because of this, we have major water damage inside our north wall. Our windows have slats around between the glass and the frame. These just snap off so you can inspect inside the window frame. Check these bottom inside corners. It may be your problem.