Taken from the Spanish Citizens Advice Bureau:
Holiday home owners should not register. The padrón is only for habitual residents of Spain. Registering could have tax implications as the person inscribed is stating that they habitually reside in Spain.
Q. Can the town hall send the police to my home to make a check on those residing there?
A. Yes. When there are indications to doubt that the residence will be established in the municipality, or on any of the data declared by the citizen, before proceeding to register or the modify the data in the Register, the City Council, will order the necessary processing acts to verify the veracity of the data recorded in the request, issuing the corresponding resolution.