Upvote:3
It depends. It's not the same a rural area or a city. Not the same the north or the south. Not the same if your motorbike is very attractive or not.
In my experience, you won't have any problem leaving your motorbike alone at night, but any safety measure will be welcomed. I've gone through all Spain (except Galicia) with my ATV without any safety measure except a typical padlock to dissuade oportunist thieves and nothing happened. The most common "problem" is to find that the bike you parked in the middle of the town square is the children attraction of the day.