this horse was darn near 3 legged lame. He is older though so he could have been exaggerating
lol, some of the smarter ones actually fake lameness to get out of work! Mine does, I have on occasion brought him up and noticed he was dog lame, been unable to find a reason, stuck him back in the paddock, only to watch him bolt around like an idiot, perfectly sound. Now, if he starts off a little 'off', I work him through it and see if he's REALLY sore or if he's just trying to get out of work. He'll quit within 5 minutes if he's faking it. He's only been genuinely lame a couple of times and both those times have been because of things I can see on the outside of him.
Some horses show very few signs of pain when they ARE hurting, though. One horse I used to know only ever showed signs of being 'off' when he was in absolute agony. The only time I can remember ever having seen him in pain was when he got an abscess, the day before it burst through his coronet. He wasn't mine but I knew him well.
OP's horse could therefore, in theory, be in quite a bit of pain, just hiding it. If you think about it, it makes evolutionary sense - the weak, the sick and the injured are the ones that get eaten, right? So if they hide their pain they're less likely to be eaten.
Or, he could be like my filly, a sensitive little flower that can't cope with any pain at all. Or he might be being a snotface.
If you trot him up, someone might be able to spot something [most lameness shows more in the trot than walk or canter], and if not it might be worth flexing the joint in question and holding it flexed for 30-60 seconds, then trotting him off as soon as you let it go. He should take a few lame steps to start, that is normal, but there comes a point where the amount of lameness will indicate an abnormality. It is best compared to the "sound" leg as most horses will differ from one another in what is 'normal'.