One of my children was involved with youth theatre several years ago, and there were two girls in the group (that I know of) who cut themselves. I know this because I'm close to their moms, and because my DD knew and she told me. Interestingly, both the girls were the high-achiever types. I don't recall anyone actually seeing the cuts - though I wouldn't necessarily know myself if the cuts were visible. What I'm saying is that NK could have somehow hidden the cuts even in a close setting like theater.
Doesn't really matter, I suppose.
I'm thinking the "cutting" of Nicole was a heinous step in NK's own self-cutting. It's like she used Nicole to damage her own self - proving her sense of worthlessness to herself and the word. (Armchair analysis there and worth what you paid for it.) In a way, the murder was a suicide. Her life as she knew it is over.
JMO.