Jake saw the hollow stalking up behind his coworker and shouted "Shelley! Behind you!"
Shelley fired blindly, in several directions. She was just a normal human being, unable to see the hollow in any direct manner - aiming wherever she saw indications of movement or areas of solid darkness large enough to conceal a human-sized threat.
Notice that she fired all six shots straight ahead - chest-height for her, mid-calf for the hollow. Had she actually perceived the hollow (not even actually seeing it, but surmising its position relative to her), she would unquestionably been firing at an upward angle.
More to the point: If she had perceived the hollow in the slightest, she wouldn't have stared so pointedly at Jake as if he were a complete blathering idiot - feeling like an idiot for emptying her revolver for 'no reason'.