It is actually used for Pinterest domain verification and not Instagram. Not sure that helps.
I already looked in the header.php file of the theme – deactivated the plugins and still cannot find the source. Only way is to download the whole wp-content/themes/austin and wp-content/plugins folder and then use a text search tool (a few text editors) to look for the string. Also pls check the /wp-content/mu-plugins since I see a “Must use” plugin named System activated – never seen before on any other installation.