halcyon storm
every now and then mobs could maybe just stay still and face a certain direction where herobrine is. they would just stare at him until he disappears. if pets like wolves were affected by this, they would growl or whine.
next suggestion is maybe a ghost village. this isnt like the normal ghost village but instead a normal village but without anything spawning in it. no villagers, no cats, no iron golems nothing. all the homes would be empty, there wouldnt be any furniture. its just a dark empty room. some of the houses in these villages would have redstone torches inside them. these structures would imply that herobrine did something to these mobs.
fiery schooner