Hock and back pain, how to tell?

When I had a pre purchase done on my horse back in march the vet said he was mild positive on his flexion tests, and slightly back sore at rest and on the lunge line but showed no signs of pain or discomfort undersaddle. Does this make any sense? I mean I've been riding him with no problems, how do you tell if your horses hocks are sore when your riding him? He's not lame or anything, he never bucks or does anything to show he's in pain. I've been doing a lot of back stretching and lifting with him, which I do think has helped...Do you think the hocks or back could have anything to do with his previous owner being "big" but she never rode him that much anyways... He doesn't seem to be in pain, I've palpated his back and he doesn't flinch or anything and he also hasn't shown any signs of his hocks bothering him, what do you think? I do stretch his legs a lot, use liniment, and i think he's slightly cowhocked if that would cause pain? He's a 7 year old ottb gelding.