Code showing on every content page
If it's not in the source, it's in the DB. At a guess, this is code that someone entered using the PHP filter, but the PHP filter is not active so it's showing up as text.

As to where it is in the DB, there's many possibilities. I'd use brute force to search the DB in its entirety. (That's easy for me to say as I have a copy of SQLyog to hand. I'm sure there are other tools that provide this functionality, though: I think PhpMyAdmin has it.)

Addendum: looks like it's in a custom pane, so the table panels_pane is the most likely culprit. Ideally you want to find how it's getting inserted onto the page. Do you have the module Panels Everywhere enabled?