Sorry to say this guys ... but its a hack in this site*
Its similar to a new trend of iframe exploits that are currently doing the rounds, its designed to be random in order to allow the exploit to last longer as its so much harder to track when there is no way to reproduce its appearance.
Check your code carefully guys.
*easy way to prove it is, disable and advert service services you are using so that ALL content is provided by this site only (will only take a couple of hours to proove that i’m right or not)