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Most likely the top seal has a tiny gap, water comes in, water comes out.
m.b.s is there an opener window above it?
i agree with GQC,i had one that sounds very similar, no spotting ,no probs on any other windows on house just the upstairs bathroom,big run almost in the middle of the pane it took about 5 or 6 cleans to stop it but it did stop in the end.I picked up the house next door to it this week ,same design of windows ,same age and guess what yup same big run on a rear upstairs bedroom : oh well