However, I took this picture a couple of days ago and the "scar" has grown from dime sized, to this:
Could this be rain rot? It goes further down his spine (towards the withers). I will try to get a better picture of it today, and I plan to ask my trainer about it tomorrow. I wash my pads regularly, and only use it on him (to my knowledge, I do ride at a very friendly boarding barn where tack gets swapped often...and not always with permission).
And before anybody asks me how I could not notice...well he's kinda tall...and I can't really see over his back. I just brush his back off and go usually.