The law in Spain requires us to keep your passports on files for the spanish authorities. We need to keep this information for 3 years. 

You can read more about this on the official government website: https://www.interior.gob.es/opencms/eu/servicios-al-ciudadano/tramites-y-gestiones/seguridad/registro-documental-de-viajeros/partes-de-entrada-y-obligaciones-de-registro-documental/

Here is the same information in English: https://www.spain-holiday.com/rentalbuzz/how-to-registering-guests-with-the-police-in-your-holiday-rental-home