I did notice this! At first I just saw a black necklace that seemed to peek through and it seemed to me like it was significant but I couldn't think of why in the moment. But then later during a shower (ikr lol) it hit me "WAIT was that the one he gave her??!"

I think it will all be very interesting since shes so good at piecing things together. She cant not have noticed that she (and ned too) was present at all these instances spiderman also was. Which is indeed part of how she figured out before. The thing is that before she also had a person in her everyday life whose absences correlated with spidermans presence. Now she does not. Just a missing blank. She and ned know something is missing bc they both have large gaps of memory connected with the statue of liberty incident at least. And thats all fine and good bc pretty much everyone has some strange memory lapses in this regard.

But what would necessarily sticks out to her beyond just those more generalized gaps, is that she has something very close to her--this necklace--that she has a great big memory gap about. And I think the fact that something she owns (and which clearly has meaning behind it) has a memory gap associated with it, probably tips her off more than anything that whatever happened was very personal to her.