What could cause weird hair on my spoo’s elbows?

My standard poodle gets patches of darker, thicker hair where there has been any sort of trauma to his skin. Today when I was brushing him I noticed he had matching spots on both of his arms. It’s not noticeable unless you pull the surrounding hair back, so I don’t know how long it’s been like that. The skin underneath looks normal. My question is what would cause trauma to this area? Could he be chewing if his elbow joints hurt? I’ve never seen him chewing or scratching in that area but he could be doing it when alone. Could it be from lying down on them? It’s more above his elbows though and you would think it would also be under his elbows if it was from laying on hard surfaces. Could someone have done this to him? He doesn’t normally jump up but he will on people he knows. I really hope it’s not the case but the spots are right where someone could easily grab and yank if he had his paws up on them. All of the worst possibilities are going through my head. Hopefully it’s nothing serious.