It's something that used to be much more common, based on the idea that you signal to overtake, so as long as you're in the 'overtaking' lane you continue to signal. Stupid logic, but there you go. Not sure if it was ever taught or thought of as normal ettiquette, but it certainly isn't these days.
Some people try to use it to make cars in front of them move over, like flashing someone, usually entitled idiots who will also tailgate and jump past at the slightest opportunity, with no regard to traffic flow or other road users.
There's no special significance to it, and it's neither required nor expected by other drivers.
Most likely the noise in the French car is so loud they can't hear the blinker clicking. Unless it is for changing lanes, use of the blinker is an aggressive move. I've seen it used in many countries including CH & UK...
I'm with Ace1, it's not so much a passive aggressive signal as just to signal you are overtaking... and still overtaking if you happen to pass 10 cars.
Still the Code de la Route, the French rules of the road says you signal before changing lanes and then signal before moving back, not for the whole duration. Whatever you might think of French drivers they're usually pretty good on motorways at staying to the right and not hogging the middle / outside lane.