Occasionally I hear a low zooming sound, followed by a splashing sound and see an orange mist around my character. What does this signify and how is it triggered? It's not me drinking estus, but similar in the shimmering sound. It's not me getting a message rated either. I'm not sure if it affects my health, FP or stamina. I don't think it is a spell hitting me either. If I can correlate it to anything it is after fighting/defeating an enemy, but I'm not sure.
There are only two things I can think of that would create this effect: Either you regained some health for killing an enemy because you're wearing the Ring of the Evil Eye, or you regained an Estus Flask charge by killing an enemy or invader.
Every now and then, killing an enemy will increase your Estus carge by one, but it can not exceed your total amount. The mechanics for this are unknown, but from my experience it seems that the more difficult the enemy is, the higher the chance of regaining Estus. You also regain two Estus charges by killing an invader.