Powersounder only goes off either through water getting into the plug, (rediculously its sits directly under scuttle drain!), or batteries playing up.
Been down this long road recently, replaced with new powersounder, still went off with no hazard warning lights flashing, so not bonnet switch or alarm system, could only then be one thing, water getting into plug, even though its got silicon type serated seals to supposedly stop water getting in. Went off when car was washed, especially when cleaning scuttle, or went it snowed last.
Eureka, spoke to my vaux dealer, who told me if they replace the powersounder they also put new wiring loom and plug on. Not a vaux part number, you have to ask especially for it, comes over from germany as a make your own loom kit, solder all joints and use shrink wrap to insulate the joints. I then used black silicon sealant to seal up around the scuttle seal grommet, then fitted large shrink wrap to the underside of the grommet tube to create an extended drain away pipe to do away with the water to the side of the suspension turret, instead of the water cascading onto the plug, now its bone dry, and siren not gone off since after heavy rain and melting snow.
Another essential omega mod.
Could also be confused with dodgey batteries, which maybe it isnt, definately worth looking at or considering.
If no hazard flash, its either siren batteries or water ingress in plug!!