Exactly my experience. I've already fixed a few of these so called mysterious leaks by replacing every single part in the order listed in the HUD even if each one is listed green. The wiki page which refers to the known bugs is very vague, and doesn't account for the possibility that "fully repaired" helis might be ignoring a bigger bug in which a partially damaged heli might be read as fully repaired. I noticed that the fuel problem typically follows a rough landing in my case, which slightly damages certain parts of the heli but never gives the slightest clue as to which it is. My solution is to fix parts in groups before checking for leaks, first the engine, then the rotor, then scrap metal and finally glass, and this chases the leak down all the time. It's highly likely that all they need to do to fix this mysterious fuel leak dilemma is to fix the HUD.