So even if anybody'd noticed Osmand acting weird around coins, it's doubtful they'd make the intuitive leap--and even once John HAD made the leap, the Ugly Bird would've been the end of him if he hadn't had a very unusual old-fashioned habit about stringing his guitar.
That's a pretty common theme in John's stories: folk wisdom and old traditions are useful, but only through mindful consideration and not by following mindless superstition.
There's a story later where he combines folk magic with acoustic science to banish something.