It’s not really a “file,” its XML content generated by the Geolocation plugin at that URL. The real file that underlies it is inside the plugin’s folder, but that’s probably not your problem.
This stray character looks like it’s appearing at the start of every page in your site. Most of the time things are loose enough that it doesn’t make a difference but XML is very strict so a stray character at the beginning causes a failure.
It’s quite likely that it’s a plugin or otherwise an edited file to blame, with a plugin the most likely, so I’d start by deactivating everything else other than Geolocation and seeing if that helps.