Anyone notice insane hp regen for no reason??

Sometimes in game I notice, out of nowhere, my champion's hp regen goes crazy and I get like 30hp regen per second. Now I know what you're thinking....Ocean drake does that....but no! This happens WITHOUT having an Ocean drake....sometimes even when I have no dragons at all I notice it! I also just noticed it on NightBlue3's stream today. He was playing a Kayle jungle game. He tower dove a Graves and got down to about 200hp and then walked away from the tower. Within about 8-10 seconds I noticed he had damn near full hp again, so I looked at his hp regen and it was 28/sec for a quite a long time...until he was full hp again. What the heck is this?!? Cuz I cant figure out what the hell gives it to you!! Its like its a bug or something that gives you the Ocean drake benefit without actually having one. Anyone who can give some input on this would be appreciated b\c its been bugging me for months now! I wish I had a clip or something to show exactly what I'm talking about, cuz I notice it happens basically in every single game. I cant think of any masteries, items, or runes, that does anything remotely close to this either. Which makes me think its a bug