In my youth ( 40 -50 years ago ) this species was very common but now it had been already about 15 years ago I saw them in the wild in the Netherlands !
I've seen them a few times this year at different locations. Not as common as the white wagtail, but certainly not a bird I would consider rare. I'm really surprised you had no sightings of this species for 15 years!
I"m also myself surprized because this year I saw it already at least a dozen times at at least 5 different spots !! I realy don't know where they were hiding the last 15 years.