As a reader, I can't stand long footnotes, so I'm not just about to commit a sin, but a sin of hypocrisy.  I imagine James was helping Sirius prepare to go into hiding himself the night Voldemort showed up, and thus his absence preserved his life.  Oddly, I never liked fics that combine one or both of Harry's parents surviving and Harry sorted into Slytherin (though there are exceptions) because the reason Harry had such a profound urge to prove himself, the ambition that almost got him sorted into Slytherin, because he was raised by people who so manifestly didn't care about him, so that's a second hypocritical sin on my part.