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
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
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
X-UA-Compatible would likely get you your answer quickly.