Hi ajw.
I'm afraid I can't help you with 'why' the money was taken out of your account, but I do agree that all of our Spanish bank accounts seem open to this kind of abuse.
We were advised to try to keep the balance in our account at just above the level we expected to have taken out for mortgage payments, etc. That way if anyone tries to take money without notifying us, the bank will need to speak to us before they will let our account go overdrawn. This might seem a bit underhanded, but hey..........most of us are not used to having strangers dipping into our accounts whenever they feel like it.
WW