So far, apart from the flashing indicator lights, all I have are the relay contacts which close when the alarm should sound. Probably the boot is the best place for the sounder, in the battery box, as it is protected by the boot lock (for what that’s worth but obviously this will not be Fort Knox.)
I did think of an MP3 player that would randomly play vicious dog noises or similar. Adafruit make a player which when controlled by an Arduino would do the job. - Just a bit of fun!
Alarm software & siren
Download a pdf of the software I wrote for the alarm/indicator/hazard functions here.
As always, I don’t make any claims for the efficiency of the coding or its style. All I can say is that it does seem to work!