Would not surprise me in the least if the true “unknown’ rate for US sightings under Sign/Grudge/Bluebook would be about the same as the French data analysis. I have pointed out in a couple of threads over time that Hynek
discussed this in his writings after his split with the Air Farce and the trigger was the Michigan “swamp gas” story. That really set him off, since all he’d done…he always said…was give that as a “maybe remotely possible” preliminary hypothesis and they blindsinded him by putting it out as “The” scientific explanation. Made him feel like a jackass.
It was from this that later he described their evaluational methodology, perhaps, more accurately, their modus operandi. They would cook the books by “massaging” these cases along. An investigation would typically end up, alluded Hynek, as being something with “possibilities” as a solution. They would then do their initial write-ups taking the “possibility” of such being the solution, and turning it into the “PROBABILITY”. Playing semantics with it. Then the final report would almost inevitably DROP the word “probable” and leave the impression that such and so absolutely “was” the solution….yet NO new data or evidence had ever been added to the mix from the time when the sighting evaluation merely listed a “possibility”. Hynek stood by this allegation till the day he died, and the implication is clear: all these AF
“solved/unsolved” percentages are totally bogus. Worthless.
I have always thought, because of that, that the true percentages of
unsolved cases should be a lot higher than what the Air Farce claimed, and these numbers out of France would seem to bear that belief out.

I have always doubted the claims made by the air-force (and also by many ufologist) that the majority of UFO sightings are caused by mundane events and that only a tiny fraction defy explanation. In fact I suspected it was the opposite - most are unexplained. I believe the majority of people try very hard to find a rational explanation for anything unusual they experience. Which means witnesses have to be seeing things that are pretty perplexing if they are reporting it. It would seem that the French study confirms this.