In Frankenstein, what is Justine’s one piece of evidence against her for which she has no answer?

On chapter 8 of Mary Shelley’s
Frankenstein, Justine Moritz is accused of murdering little William
Frankenstein. William is Victor’s young brother and, by default, Elizabeth’s cousin
(although she explains that she feels as if he were her brother as
well).

Justine had no alibi. In fact, the night of the
murder she was given time off. She then went to visit an aunt in a nearby village.
Therefore, nobody could have known exactly what Justine was doing that night.  However,
the one piece of evidence that Justine could not explain in order to proof her innocence
was how the child’s locket ended up in her possession. Justine had placed a locket with
a picture on William’s neck hours before he went missing. The fact that she had it in
her possession after his death had occurred made the jury conclude that Justine must
have been near the dead body. Yet, not even Justine could explain this occurrence and
begged the jury to consider her character and the flawless way in which she has always
lived her life as proof that she would be incapable of doing such a
thing.