okay, i give up

In today's stage of the Tour de France they passed by some people doing a sort of synchronized flag-waving dance not too far outside of Antwerp. The flags in question were France, Netherlands, Belgium, and ... something I didn't recognize. A white flag with a red and white striped shield and a red fly end. (Argent per pale dancetty gules? On an escutcheon gules three pallets argent? Something like that? Vaguely?) I can't find it anywhere in the Googles and it's annoying me. Anyone have a good knowledge of the flags of the Low Countries? (It's not Flanders and it's not Wallonia. And it doesn't seem to be any regions of the Netherlands either.)