i dont think its a seal, but maybe a rage he has inside of him as stated b4. it can be a foreshadowing but i would think they can have a better way to do that, maybe a facial expression? having his eyes almost turn completely bloodshot is a lil too over the foreshadowing border. he hasn't needed to use it yet wether its a rage or seal or it might not be ne thing at all. he only fought naruto and his healing powers were enough. and the sannin were busy amongst themselves. the times with kakashi, a mission was occuring. he had to put the mission first and fighting would be pointless. if it is something, maybe it will be explained soon, like the sound four failing to bring back sasuke and then oro sends kabuto or something. or maybe it was just shit.