For that twister my wife and I were actually in Phillips Arena watching a pro basketball game. I had received free tickets and, though neither of us are pro basketball fans, we decided to make it a date night. Next door at the GA Dome the college SEC basketball championships were being played. The tornado ripped the canvas roof of the Dome and it was evacuated. In Phillips Arena there was not even an announcement made. After the game we walked out of the arena to encounter the flashing strobe lights you mentioned, with no idea what was going on. Downtown Atlanta looked like a war zone. It was quite surreal until we got to our car and flipped on the radio.
By way of explanation, the strobe lights were interior fire alarms in the high rise buildings, visible from outside because a huge number of windows had been shattered.