The present invention relates to an emergency notification system where indicators are mounted on or in a building in such a way that optical or thermal signals emitted from the indicators form a time-variant indication detectable outside the building of an emergency event taking place inside the building. Sensors detecting a predetermined sound are mounted inside the building and are each connected to a nearby indicator. When a sensor makes a detection, it sends event information to its associated indicator which will prompt the indicator to emit a first optical or thermal signal. Based on a signal from a timer connected to the indicator, a property of the first signal will change after a certain time has passed, thereby providing the time-variant indication.