Have you jogged him to see if he favors it? Have somebody trot him on a long, flat, hardish surface and watch to see if his head bobs as a certain foot hits the ground...that is the indicator if he is lame. On a front leg the head will go up as the lame leg hits the ground. Don't ride him...I would call the vet. It sounds like it could be a mild bowed tendon, which can be very serious especially if untreated...you might need to wrap the leg or hose it or poultice it. Odd that he's not in pain though. Did you ride him recently, or did you see him running around in the pasture? Or is it muddy right now in your pasture?
He knows when you're happy; He knows when you're comfortable; He knows when you're confident; And he always knows when you have carrots.