I have a filter set up on a view in Google Analytics to show the full hostname for pageviews. This enables us to analyse traffic on our various microsites. However I'm also seeing pageviews recorded against external URLs e.g. www.youtube.comm.youtube.cominst.webinstantservice.comI could filter these out (or just ignore them) but I can't figure out why they're appearing at all since they aren't using our tracking code. 
External sites appearing in Google Analytics
google analytics;referrer
Check to see if you are tracking your YouTube channel visits in Google Analytics. This would have required you to configure your YouTube channel with the tracking code. That would explain the www.youtube.com and m.youtube.com hostnames. You would also see these visits landing on pages like /channel/.....The inst.webinstantservice.com hostname is spam referrals from rank-checker and monitizationking.