Agree with
@Marcus-UK with an addition that the care unfortunately may become of interest to "bad people" (probably faster than the police).
Granted it probably may matter not if its a paid "secure" garage\parking area, but IMHO its still better that just "leaving it in the open".
Any time I have to leave my car anywhere for extended period of time I use paid parking facility. Don't rightly know if they have the same in Spain, but here in US I even went as far as to use the facility where you leave the keys with the car (obviously make sure that anything of the value is out from it). The attendant will park it inside the facility and nobody but authorized personnel is allowed into the area...
Good luck,
@Luka and Buen Camino! Don't forget to post on FindPenguins - will be following you!