Micra auxiliary battery alarm problem

I had a similar thing happen on my old Saab 9-3. The error codes were all over the place, and the alarm was completely dead. Turned out the auxiliary battery was shot. How old is the Micra's battery? Might be worth getting it checked.

The battery is probably quite old, as I haven't changed it since I bought the car three years ago. How can I properly check the battery? Is a multimeter required, or is there a simpler method?

A multimeter is helpful, but given the age and the symptoms, I'd just take it to a workshop. They can do a proper battery test and tell you for sure. It's probably just old age, but they can rule out other problems.

I followed your advice and took it to a local workshop. The auxiliary battery was discharged due to ageing. The total repair cost came to 330€, including a new battery and labor. The alarm system is now functioning correctly. I appreciate your help!