My misses mate just had similar.

She banged her car up drink driving years back. Banned for a good few years so parked it at her flat and it has sat ever since. the other month it vanished (a few months before she was due to get it back on the road)

The car wasn't registered to that flat as she had only just moved in when she bumped it, the flats belonged to a friend so wasn't registered as living there and pretty much nobody knew it was her car.

She found out who took it and they had to pay her for the car in the end to avoid 'unwated complications'.

Not to say this is the case, but if it is parked on private land then I would think it is theft when you actually take the car.