Finding where a snippet of code is coming from

I recently noticed that a site that I work on is making a call to an outside website on every page load. The following code is showing on every page:

<script type="text/javascript" src="http://ift.tt/1FZj9Fx"></script>
</head>

As you can see it's at the very bottom of the head. I don't think it's malicious as it just loads a 404 and no malware scans pull up any red flags. I can't find the link in any of the source code, I also can't find the link in the database anywhere (by searching for "overton" in phpmyadmin).

I've disabled every plugin on the site, and the code is still there.

I'm out of ideas on things to do to hunt down where this code comes from. Any ideas or other tactics that I can take to hunt the source of this down? Thanks!

(I'm also a little unsure of the correct tags for this question, feel free to suggest others)