You can use the Sync with active document button to track the current active document. If you don't see this button, the active document and solution explorer will be automatically synchronized and the icon will be hidden. You can to uncheck the option ('Track Active Item in Solution Explorer') in the options to sync the active document manually. 

If this icon is not there, Go to Tools-Options. 

Find the 'Projects and Solutions' on the left menu and uncheck  'Track Active Item in Solution Explorer', click OK.