Our family car, for instance, sports a sticker of this flag which we purchased for a modest price when visiting a local stately home- and plenty of other vehicles do the same. I even bought a big actual flag of this - on impulse - then didn't know what to do with it, so ended-up by draping it on the wall of my office.
Since this is not any kind of campaign or political movement, I can only suppose it expresses a general but seldom-articulated love of the county of Northumberland - which is indeed a county that is easy to love.
The effect of seeing this flag unexpectedly, as I go about business in the city, is to give a lift of the heart and a breath of cleaner air - it is a small and secretive ray of hope.