Nothing new has been added to the vehicle in the last year that might drain the 12V battery, Is it just a sign that the 12V battery is on the way out?
Quite probably. Four years plus an unknown time in storage (2020 was the height of COVID after all) is a reasonable lifespan for a "nothing special" lead acid battery. In winter, lead acid batteries perform less well, and are more likely to die in winter. Though this is more visible with ICE cars, as cranking a cold engine is harder, and cranking is a very heavy electrical load to start with.
On top of that, I have a suspicion that the Mark 1 (pre facelift) ZS EVs don't charge the 12 V automatically when it gets low in voltage (and hence charge).
Your photo shows the auxiliary (12V) battery between 13 and 14 volts (it's an incredibly crude "voltmeter"). That says nothing at all about the health of the battery. The DC-DC can slam well over 100 amps into the auxiliary battery (needed for some 12 V loads like power steering), so even the strongest battery will jump up to this voltage range immediately. Weaker batteries will change voltage (under more moderate charge current or under load) more quickly.
You can get a feel for the state of health (SoH) of the auxiliary battery by letting it stand with no load (preferably remove a battery cable to ensure this) for at least an hour, then note the resting voltage with a multimeter. A good battery will measure about 12.6 V or higher (assuming it's been fully charged). A very poor battery might drop below about 12.0 V due to self-discharge.
You might get a few months more life out of it by giving it a good (e.g. overnight) charge from an ordinary 12 V car battery charger. Being winter, you might not be using the car as much, so it might just not be recharging fully on each drive, and as I said I suspect that it's never charging the 12 V battery unless the car is in ready mode, or the main battery is being charged (this will also charge the 12 V battery).
