I, too have been seeing this... and wondered what it is. Mystery, at least part of it solved. The 'symbol' signals the presence of a 'Bluetooth sound device'; aka, speaker. Disconnect the Bluetooth connection...symbol leaves and screen clears.
Not the best solution... especially if you're using this to 'hear the audio'!!! But, under the current circumstances of the 'Bluetooth conflicts on the Android 4.3+ version', this may be the only way for a 'clearing' solution. Hopefully, both Google and the Droid programmers will get their collective 'repair on' and fix the problemS!!! Soon. Hope this helps. -net500cg
Yeah, that happened to me also on my Nexus 7 when the YouTube app recently updated. That icon is the pairing icon for a TV on the same WiFi network. I had use this feature a number of times before the update, but when YouTube updated I didn't realize it. For the next few days I looked for a way (or settings) to remove this icon and restore YouTube back to normal. The TV in my bedroom is connected by WiFi, I figured let me see if I get the same icon in the middle of the screen on my TV, the problem was not affecting the YouTube program on my TV. I correctly turned this feature off on my Nexus 7 and the problem is fixed. Check out this link:
Oh Yeah, I just got the Note 3, so I was just looking over all the great info here. I haven't activated yet but I will in the next few day. I'm looking forward to being a part of this forum. Peace.