Stealth and disappearingFollow

In a recent fight with another roque I came up behind him and crit ambushed him and he almost died, so he started to run away. Then all of the sudden he disappeared and i couldn't track him anymore. How are we able to do this, because i was under the impression that when we are in combat we cannot stealth. If there is a way to do that i would love to know cos it would be very helpful.