Look at the bottom of the window on the outside and see if you don't have some small weep holes. They may be covered with little plastic "hoods." These can get clogged with dust and cause them to overflow into the inside. A good
cleaning with weed wacker cord can often solve this problem. This may not be the cause, but it's sure the easiest to start with.
Even those I know this, even I had this problem recently. First thing I did was check the channel, and even though it seemed dry, I cleaned them anyway. Still leaked. It wasn't until after I got home that I realized that I actually had a dual problem. The side emergency window gasket was a little old and dry so it allowed too much rain in during a series of storms. After I cleaned out the weep holes again and it still did it, I felt like it was finally time to remove the window and redo it. I also decided to replace the gasket while I was at it. That was when I
finally realized that there was a second set of weep holes on the emergency windows
. A new gasket and a thorough cleaning of the second set solved my problem. I used a hose with a sprayer on it and no leak!