I've noticed these marks on my 3yo upper arms and back the past few days.
Seems to come out when he's in the bath but is not in line with the water level and the water isn't very hot.
It's on both sides and like a line that goes round his arm and across his back.
At first I thought it was from carrying a backpack that was tight or rubbing but I've been with him all day and he's not worn one today!
No discomfort or anything when I ask him if it hurts he says no.