no actually it is due to this:
During events some times players tend not to kill bosses completely at all, bosses may have spawned in some irregular locations in the map and hence players must have missed them. He must be circling the map or some low lvl player must have seen them and must have called him to kill them.
This happened to me once in elkenver, i went to kill a snowman and when i reached the 3rd snowman near the southern part of the map there were 2 greybones there and 3 statues. lol i had enjoyed killing them
