This has now happened to me 3 times, and I’m pretty annoyed as well as curious.
I have Comcast digital cable with DVR. While watching a recorded program, suddenly I got green screen with words to the effect of “Stand by for an important audio message.” This was accompanied by a horrible screeching pulse sound. The only way to get rid of it was to turn off the cable box. I was then able to resume my show where it had stopped.
Depending on the manufacturer of your box, it is probably a software glitch in the cablebox. A certain company who makes such boxes has had a notoriously bad time with digital cable protocols. But yes, it is basically the cablebox thinking it is receiving an emergency alert system message.
Do you have more than one cable box? Does it do that on all of them? I’m thinking it’s not so much the TV as it is the cable box. You could probably just swap it out at the cable…uh…store?
The only way I could stop it was to turn off the cable box. And when I turned it back on and replayed the show, the warning was not there. So if it was a warning, it was live. And if it was a warning, it’s interesting that I never got one until I got the DVR box.