There are actually two anal sphincters, plus some valve-like folds in the anal canal between them. The valve-like folds ensure that when you pass flatus (that's the technical term for gas or fart) it's only flatus that escapes. Is the external sphincter airtight? That's the one under voluntary control, so probably the one you're asking about. I don't know for certain, but I doubt it. More likely, the small, continuous "leak" is just too insignificant to notice. Think of a beachball with a tiny pin hole in it. You can still fill it with air, and it'll hold most of it for a while, even though a little bit is always escaping. The more deflated the ball, the less pressure is pushing toward that hole, so the leak slows. Though, I'm not sure if there is a definitive answer. Then again, there are some obscure anatomical journals that publish a lot of weird things you never knew anyone wanted to know.