The only cause for that I can think of is the windshield is delaminating which is rare but happens. If it really is that and moisture is forming between the laminates the only fix is to replace the windshield.

Another thing you can do in the meantime is to leave a note on the OUTSIDE of the windshield with your parking stickers number, if there is a number. OR just say on the note that you have a parking sticker and please don't tow the vehicle if the windshield is too fogged to see the sticker.