Anyone know what these dark streaks are on my son's sports tops? He's got several of the same top, and all the darker coloured ones (even only slightly darker blues) don't have them at all, but the orange and light blue do. They seem to be only on the front of the tops from armpit areas and down the sides, not too much down the centre. Material is 100% polyester and washed on eco 40 with Formil bio liquid detergent. Barely faded after a wash.