This is a photographer's journal
She looks frightened, as if caught and about to be arrested. A cop said 'Come on lady, put your hands behind your head.', and she complied. The headlights of the squad car beam on the upper right, ready to take her away. Tough town that Beacon.
I was thinking the same thing about the look, John! Though my less film noir interpretation was that she was just afraid she was in an unflattering pose (though she needn't have worried).