Did you have a Chromecast set up as 'Master bedroom' previously? Maybe it's just stuck in the Youtube app's cache. Try going into your Settings >> Application manager and doing a 'Clear cache' for the Youtube app.
Odd that you're only seeing this with the Youtube app. If the above didn't make any difference, if your wireless router has any kind of 'connected devices' option in its settings page, take a look to see if there's anything showing for that 'Master bedroom'. Any Chromecast you've paired up with your home network will get an allotted IP address so at least something should show up. I'm aware your other streaming content apps should also see this mystery Chromecast but there has to be some kind of trace of it in your LAN to even show up in your Youtube app.
