Alarm sounds when 12V battery is disconnected

[alohart]

When preparing our i3 in long-term storage recently, I wanted to lock the doors without using the emergency key because I want to avoid popping off the plastic cover over the driver's door lock as much as possible. Our parking space has a wall on the passenger side, so I could not exit through the passenger door. So I locked the doors using Comfort Access and then disconnected the 12V battery. I was initially surprised that this caused the alarm siren to sound. This makes sense to prevent a thief from disarming the alarm by disconnecting the 12V battery.

Without the 12V battery being connected, what powers the alarm siren? Maybe power is provided by a rechargeable battery, but a battery isn't shown in the parts diagram, so it might be integrated into the siren. Rechargeable batteries don't last forever, so can I expect to have to replace the siren eventually when its battery fails?

 

[Buzby]

The battery will be an anti-tamper on the siren itself, when it runs out, it will fall silent. On reconnecting the 12v circuit, the siren battery will be re-energised and recommence monitoring as before.