I lost one a few days ago, after a quick turn. Arrgh! I make it a habit of feeling them, and I think (could be wrong) that I did so in this case, so maybe it just wasn't turned the full quarter turn...
Anyway, the anti-siphon valve worked fine and I never noticed until I ordered fuel the next day at my destination, and they discovered it.
Aero Air was able to send me one early the next day, so it only cost me a day... and $965 for a fuel cap!
I'm planning to order another one and keep it in the baggage just in case. I figure it's insurance, and if I have one, I will never lose another one!