I don't think wizards can detect magic that easily. Patronus is definitely easy to detect, it radiates both in visible light and in... emotional waves, whatever that is ;)
But think about how it's hard to detect someone who is polyjuiced, or someone who is imperiused. Or how Dumbledore had to cast spells to detect what transfigured objects did Harry has on him when he looked for Hermione body.
It seems perfectly plausible that wizards can't detect (without actively casting some detection spells) a transfiguration that happens on the microscopic scale and can't see.
I am assuming that Voldemort, about to attempt the subversion of a prophecy to destroy the world, is not standing around in a graveyard with Harry Potter, a recently reanimated Hermione with unicorn powers and a bunch of Death Eaters of at least slightly questionable loyalty without any detection spells raised at all.
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 114, and also, as a special case due to the exceptionally close posting times, chapter 115.
There is a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)