I'm doing the bit where you find the assassin of old along the dark brotherhood line and when i get to where he supposedly is, he isnt, I mean he's not there, just an arrow that indicates where he's supposed to be.

Now either he's not there or he's so stealthy that i just can't see him,

Has anyone come across that?

I have been to the place before and cleaned it up, could that be the reason?

He should be behind a "secret" wall behind one of the thrones. IIRC there is a handle nearby to open it.

Near the throne there will be a switch to open a door and there will be a 'assasin of old'.

I know all of that.
The assassin isn't there!

I just found this
http://elderscrolls.wikia.com/wiki/Locate_the_Assassin_of_Old

It svcks when things don't work out as expected.