To my suspicious mind, and Spanish plod might wonder the same thing, its odd that you mention dashcam and gps log but dont say that you know yourself you werent in Spain. After all their electronics say you were. Could it be a scam? Have you traced to communications origin etc?
Anyway, Spanish bureaucracy can be very long winded and old fashioned. No experience of their police but other issues dealt with at the local ayuntamineto , which is where local police are based, took forever and needed to be in Spanish.
If the fine is not a silly amount, it may well be easier simply to pay.
