Since the Libyan conflict, there have been many KC135 tankers flying over Spain every day heading to and from the Med. Mainly using the ID's of 'Gold' and 'Mobil' as well as variations on 'Mobil' such as 'Mobl' and 'Moble.' Some days there are as many as 6-8 aircraft flying over.
Not sure where they are flying from but could be somewhere in Portugal or perhaps the Azores.