why no audible alarm when flashers flashing??

My car is a Renault Laguna 1.9 DCI estate which is kept in the driveway of my house. This morning when I looked out of the window the hazard flashers were on but there was no accompanying audible alarm sound. I unlocked the doors remotely entered the car and pressed the hazard alarm switch and the flashing stopped. I have no idea as to what set the flashers off; the car did not appear to be tampered with but I am curious to know why the audible alarm did not sound.