Emergency Bumper Indicator

[Warrior Princess]

Coming from an event at daughter A's school, walking to parking lot to leave. They were handing out flyers with instructions. The parking lot guy was pointing out some sort of emergency thing on someone's back bumper as they were backing out, that gave them a warning about something behind them so they wouldn't hit it. I figured I was going to have to make a right turn to get home because of traffic.