Strange.. F3 + H can also detect on the aura of the entity.. Which means it can find hackers with Kill aura? Or is it for entities?
They are called hitboxes and they are used as a sort of target finder. It also works on people that are invisible, so it's a way to see people with an invisible potion.
It doesn't detect invisible players/entities or players/entities that are not visible to the player (Behind a wall or something).
Maybe they did something to it recently but I remember my friend and I used to do it on a factions server we played on. Edit: Yeah it is patched but this is what I was on about.