I just put on my headphones and found that a looping ad for NBC's prime time show lineup was playing. I couldn't find any source of the audio in any of the tabs I had open in Safari, even though it sounded like the soundtrack to a video. I finally had to restart Safari to make it stop. All the tabs open now are exactly the same as when the sound was playing, but the ad has not reappeared.
A bug? Or something else? I find it highly suspicious.