We have a few of those in my area (receiving a signal from emergency vehicles that is, not transmitting light status). Long before you see the emergency vehicle in the area, a weird white light turns on above the traffic light, then several seconds later all lights turn red, and then after just long enough for people to be really confused, an ambulance or fire truck will show up and drive through, and then the white light turns off and it goes back to normal operation.
The only time I had it happen while on FSD, I was already at a red light, so it was just a really long duration red with a white light on top for a while from where I was. FSD stayed put as expected