YouTube Experiments with "Live Broadcasting Animations"
I'm making up the name, but for upcoming live broadcasts (like Hangouts on Air), you may see an animated symbol on the video player. Part of the animation is captured here.
If you try out YouTube's Redesigned Player, you may see these animations before and just after a live broadcast has streamed.
https://www.youtube.com/testtube (currently at the bottom of the page)., a fellow Hangout Mastery member, pointed this out to me a while ago, and mentioned that it may be part of the "Try Something New!" updated player area in YouTube:
If you switch to using YouTube's Updated Player and want to return to the original, you'll be given that opportunity again on the "Try Something New!" area of YouTube linked above.