I discovered that wandering with Dear Miss Fletcher involves learning Genova's secrets.
And the crazy thing is that I've walked past this charity slot, from 1729, so many times. In another location, Sabina showed me where the charity holes in the wall had been removed.
But ... if you don't know what you're looking for you will never find it. In another place, she stopped me in a narrow alleyway and showed me the indentations left in the cobblestones ... by chariots.