Visiting some friends staying in a camping for the afternoon we parked on the road outside. When we returned we found a contravention 35€ on the windshield. And on the cars parked in front and behind.
Not a sign of a line, but eventually some 30m back we found a No Parking sign - on a much narrower part of the road. I reckon the Mairie are treating it as an earner.
If I was in UK I'd take it up with the parking ombusperson (can't remember the proper title). I've failed to google the equivalent in France-is there such and is it worth it?