Recently, the Guardia Civil, traffic police have acquired hundreds of unmarked cars to patrol the roads in search of people speeding or driving dangerously.
For example, they are using SUVs, Stelvio belonging to Alfa Romeo, but there are older makes and models, such as the Seat Exeo, which is no longer on sale. It’s precisely these old cars that draw less attention – sneaky, eh?
Up in Cataluña they use Citroën C4 SpaceTourer fitted with speed radar but whatever the make and where ever in Spain it is, these patrol cars operate where there are black spots; i.e., sections of road with a high percentage of accidents.
But it is no only cars, but also motorbikes. Yes, a Guardia Civil rider kitted out in his normal uniform would be easy to spot on a standard bike trying to pass inadverted, but the riders don’t wear uniform and only put on a Guardia Civil reflective jacket when they get off the bike and come towards you with a fines pad.
The 30 bikes used by the DGT are Honda ST 1300 Pan European and BMW R 1150 RT. The latter are used by the Guardia Civil as normal patrol bikes with all the markings and gear, too.
(News: Spain)

Open the window slightly. Ask for I.D. Ring Police and ask for help. Ask if they have unmarked officers in your location. Better safe then robbed or worse.
Oh yes don’t speed or drive erratically!!
Try To go around roundabouts correctly. Maybe use indicators. ??
Is it me or does the fact you are told to stop your car by an unmarked bike and the rider not in uniform who then puts on a Guarda Civil high viz to fine you seems strange. I am all for police and I know they have a difficult job but has a solo driver would you stop at night for an unmaked bike with someone in civvies?