Have you checked the rear parcel shelf ? If that is holding moisture that would steam up the back window. If it does not feel wet some silica gel packs will help remove the moisture.
In the recent snow I found the angle of the rear screen is perfect for snow to slide straight into the boot avoiding the drainage channel when the boot lid is opened
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Otherwise, no water in the boot of my Jan. 2011 S60.
Has the car been rear ended at any time ? Have you checked the rear light clusters ? They are the usual weak spots for water getting into a boot.