Well perhaps someone might have used amx_addban on him after he had left the server.
That way you will not be seen on the banlist, and also it will not show you the person who banned you.
There is one way: The highstaff have to look into the server logs and try to find if someone might have used that command on you. (or by some other kind of method)
Therefor it is important for te highstaff that you give them the exact time this happened. That will make it easier for them to find it, if it is true what i've said.
-PEACE N' STUFF