My 10 Yr. Old Has A Dark Reddish Brown Spot

my 10 yr. old has a dark reddish brown spot on each shoulder about the size of a quarter. The 2 spots showed up in a matter of 15 min. The area is not raised or warm to touch & doesn't itch or hurt. In the past he's had what looked like this on upper arms, but were streaks, also some that resembled abrasions. These marks are always gone in a couple days. No change in food, or meds. they are not bites, they just appear & in a day or 2 are gone. any idea what this could be?