Would one of the disciplines make it more likely he checked ( or what do we say for winning rolls?. Rolled high? ) for hidden messages inside messages?. Is that something that sufficiently high skilled characters might do automatically?. Do any of the casters have something like a Turing Hat, or park themselves at Benchley ?. Is the sort of skill that lets you find people who are sneaking or hiding, ( is it Spot/listen ?,) something that you could routinely use when eavesdropping?.
Going back to Maggie's inner dialogue, "any Thinkamancer who plucked her would find this note in her G-String." So if that's true, then most likely Charlie picking up the note is a foregone conclusion, given the apparent abilities of the Arkendish. If he can pluck her, he can pick it up. And the GK Thinkamancer should be a top plucking target. The question is whether he can decipher the message, or rather, given his apparent intelligence, how long will it take him to decipher it.
In mechanical terms, I infer that the notches cause some kind of variation in the natural vibration of her G-string. Heh... Why do I imagine a Star Trek style bridge full of inner-circle Archons, and one of them (a dark-skinned stunner in a short red uniform dress) turns to Charlie (sitting in the captain's chair), puts her hand to her ear, and says something along the lines of "Charlie, I'm detecting some anomalous readings."