I've been watching the Elisa Lam video and several things struck me...
She was not scared. Definitely not. Perhaps a touch frustrated when she comes back in to fiddle with the buttons for the last time, but there's no actual anxiety that I can see.
When she first goes in, she seems to be expecting someone else to join her - if she was travelling on her own in the lift, I think she'd stand in the middle in the front of the doors. What she actually does is stand in the corner by the wall - I do this when I am making room for someone else.
Darting out for a quick look seems to be done in a flirty, lighthearted way. She immediately looks to her right, so she's expecting them to come from that direction...which is not the direction she herself has come from.
When she "hides" in the corner near the buttons, it doesn't seem to be prompted by fear. She even seems at smile at one point. I think she was planning to say "Boo" to someone coming in.
When she's standing outside the lift, she seems to be preening in the mirror that's opposite. She raises her arms to lift her hair, classic flirtation moves.
I am convinced she was either playing hide and seek with someone, or thought she was.
Whether there was a actual real person there, or she was imagining them, I have no idea. But I am certain that's what she was doing.
Poor thing, whatever happened - murder, suicide or accident - she must have really suffered.