When you're an expert at something and you spend all your days looking at these things, you can pick out remarkable details from nothing. Any regular person looking at the original photo (below) would have seen nothing! https://x.com/Warkslieutenant/status/1808884139585610231/pho... (it's the picture with the curved frame on the left)
Just seeing the outline of the frame could probably highlighted it was part of the missing series, assuming that is there are very few other paintings with an arched frame.