MinecraftCC Riddle #6

[hugohuisman]

You dont recognize one when u see them.
If they are there you dont know it.
If they have been there you do know.
What am i?¿
good luck

(sorry for the aweful grammar )

 

[PD400]

Griefers?
Moderators?

 

[hugohuisman]

oke i need to work on this to make it a bit harder -.- Griefers is the right answer!