Now this says nothing about going after parts on the trunk lid but that she thought she was going for help. In an extreme situation, an irrational panic would not be unreasonable.
So she went out on the trunk but perhaps not for body parts.
If you look closely at those frame blow-ups of the Zapruder film, where their faces are touching, or almost touching, imagine the shock and horror at seeing his head explode like that as you are looking him in the eyes! She almost certainly had bits of brain, bone and tissue in her lap. As his dead body slumps, his head is practically in her lap. This is why i think she panicked, tried to get the hell out of there, didn't know what to do, etc. Complete and total shock.