SOS light flashing
This started happening after the main battery went flat enough that not even the central locking worked.
My own fault for leaving the dash cam connected on permanent feed for too long. The power supply (a branded Garmin one) for the dash cam (old Mini 0803) has voltage drop detection however the battery still went flat. To be fair it was after a good three weeks of not being used.
This is the SOS light on the red button above the interior rear view mirror.
A DiagBox diagnostics session confirmed the 'Condition of the backup battery' was 'Less than 5 minutes'.
I had hoped it was a rechargeable battery and it would recharge during use of the car but apparently this isn't the case.
The battery itself appears to be two cells in series with the following text printed on it:
Varta Microbattery
2CR 2/3 AH
920968180470
6v Lithium
They are available from the main dealer however they aren't exactly the price of an AA battery.
From what I've read, and if you are skilled enough to replace them, the cells inside are off the shelf components. Otherwise, since I replaced mine a few months ago an enterprising individual has started offering a remanufacturing service on eBay. I can't recommend them as I've not used them, search for '920968180470 battery swap'.
As the previous option wasn't available to me at the time, I took the approach of buying a cheap second hand ITU and hoping the battery would keep me going for a while. I can then refurbish the old battery at my leisure.
The official name of the unit the battery lives in is the ITU or Independent Telematics Unit. On eBay it's either sold as an ITU, Telematics Unit or Bluetooth module. It looks like the battery is fitted to a variety of different vehicles with the Magneti Marelli unit. You may find that depending on how it's advertised, and the target vehicle, the price might be quite different.
Getting to the module is challenging, I got flashbacks from replacing the clutch cable clip on the Xantia as the ITU is right up behind the fuse box on the passenger's side.
I found it easier to unscrew and pull this out a bit.
It's quite challenging to take photos of the ITU as it really is so far up.
The module is clipped to a support bracket which in turn is in turn attached to a stud on the bulkhead via a nut.
There is a cable tidy clip for the bigger cable which I found gave more access if I unclipped.
The clip for the larger connector onto the ITU is a challenge, in fact the removal of the whole unit is so persistence and patience is needed. I haven't decided whether it's easier to remove the ITU with or without the bracket, so go with whatever works for you.
If you're able to get access to Citroen Service, there is a document which is semi-helpful named 'REMOVING - REFITTING : INDEPENDENT TELEMATIC UNIT'