“An evil and adulterous generation seeks for a sign, but no sign will be given to it except the sign of Jonah” Matthew 16:4
Hmmmm......I wasn’t looking for a “divine sign” as you’re implying. It was just something interesting we saw in the painting and I was looking for background information. Da Vinci could have painted it intentionally, or it could have been added by the many many “touch-ups” done over the centuries.
It’s common knowledge that artists like Da Vinci and Michelangelo would include things in sacred paintings that are unexpected. For example, there is something small and very unflattering painted into the Sistine Chapel ceiling secretly directed at Pope Julius (whom Michelangelo despised). I forget exactly what it was, but our tour guide pointed it out when she took us inside the Sistine Chapel last year. Our tour guide was a professionally licensed (by the country of Italy) guide and had a degree in art history.