As an inhabitant of the Netherlands, I would be very surprised if we would police our border with either Belgium or Germany. There are no natural obstacles, and plenty of spaces with forest crossing the border. No-one but the military comes close to having enough man-power to enforce that border.

Sure, with actual army deployment we could get somewhere (it seems we have about 20 soldiers per kilometer of border). But the massive difference in effort between "police airports and the coast with radar" and "Mobilize the army" is huge.