Why did I just hear a siren?

The sirens (luchtalarm) are part of the Netherlands’ national emergency system.

The sirens are tested every first Monday of the month at noon and last for 1 minute and 26 seconds, except if this day falls on Remembrance Day, or any national- and religious holidays.

If the sirens last longer than the specified 1 minute and 26 seconds, or on any other day than the first Monday of the Month: go inside, close all doors and windows and tune into Radio Rijnmond on 93.4 FM or go to https://crisis.nl/ (Dutch)