So am I just missing a rule somewhere, or how do you handle extra successes when someone uses an extended range detection spell like clairvoyance?

Ironically, my idea for a house rule turned out to be the actual rule. SR4a, page 206: "The Spellcasting Test [for a Passive Detection Spell] serves as a magical Perception test, against an appropriate threshold determined by the gamemaster."

Oh cool. Well that works out well then.