this is exactly why i never clean the top frame on any window that has vents,too much can go wrong.
when i first started wfp i used to clean the top frame,untill i got a few complaints,and one customer going wild because water had run through a vent and damaged a photograph on the window sill.
for the last 3 yrs i havnt touched one top frame with a vent in it,and not one complaint since i started working this way.On downstairs windows i may give the top frame a wipe with a damp scrim now and again,upstairs i give the top frame a wide birth