Someone noticed a flash on the roof of Notre Dame a while before the fire, from a webcam. I looked at the stored timelapses (each hour gets a one minute timelapse, one second is one minute) from the webcam and found a shadow (person?) moving on the roof and a flash.
Correction: The timelapse might not be a timelapse. Look at previous 1 min movies, the shaking of the cam seems like a timelapse, but later when the fire starts, the flames don't look like a timelapse. Don't know.
The clip is marked 15/04 17:05 on the webcam-site and the flash is at 23 sek with some movement a couple of seconds before. Meaning this (might be se correction above) is several minutes in real time.
There might have been a person doing something there about an hour before the fire started. The workers had a day off. Another correction: The workers where working but closing up shop for the day.