how can i get rid of marks on windscreen?

have had car from new never owned or used a sat nav...When car is left stationary over night there are suction marks like a sat nav visible on windscreen This is annoying as i have no sat nav There are about 10 visible marks they normally show when its cold/damp.have used methlated spirits vinegar etc...but doesnt seem to shift this.Why have i got such marks? many thanks