I have an ad unit called "Top Rich Media" which seems to be doing okay. But I have long since forgotten on which site I placed it.
Ah! Sometimes I also run into these kinds of problems.
My solution, and this depends entirely how you manage your sites on you PC, is to pick something specifically unique to that ad code, and in my HTML Text Editor I go to "Search Disk -> Search Directory/Options".
Example:
Search [without quotes] "brown widgets"
Search Directory/Options c:\my-site\*.htm [in my case]
Mostly 99.9% of the time I can pick up every instance.
I use NoteTab Pro, and that feature along with "Search and Replace" - it has paid for itself 1,000's times over.
No doubt other similar text editors would have the same thing.
[Added] - If it lives inside a Server Side Include file on my sites, I search either of these as well:
c:\my-site\*.txt
c:\my-site\*.shtml
Heh! Much of my set up is 100 years old. But works!