[cheshired5]

I changed my siren in 2014 when the car was 12 years old so it's difficult to see beyond a failing siren battery being the cause.

My hazards would flash with no siren noise then sometimes the siren would sound all at the same inopportune moments in the middle of the night.
The system also became incredibly sensitive to above average air temperature/air pressure changes so a cold night following a warm day or an impending thunderstorm at any time of the day.

No Vida error codes either.

I can't say 100% that your siren needs changing or at least splitting open to change the battery but I also wouldn't spend time investigating any other reason for the erratic behaviour when the battery is 12 years old.