How to find meta tag source?
I was going to check the theme for you but it looks like that's a private theme so I'll try to outline some other places you can check.

The most common places you would find it are html.tpl.php and template.php but it sounds like you've covered those.  If it's not in html.tpl.php then it is likely being added with drupal_add_html_head().

Other places to look outside of the ones you've already checked could be in your /modules folder - looking for any custom modules that came with the theme.  If the tag is being added with drupal_add_html_head() then it could be in a custom module.

If not in template.php, check if the theme uses individual files for preprocess functions.  ie look for html.preprocess.inc or something similar.

The theme might have some *.inc files for random features - you can check those as well.

A quick grep of your theme folder for drupal_add_html_head or X-UA-Compatible would likely get you your answer quickly.