I think a conviction is unlikely because of what posters have said above. In addition, the CCTV would have to record her picking the item up and keep her continuously in view to leaving the store for a conviction to be possible on CCTV alone. If she is out of sight of the CCTV for only seconds, that is enough for her to have replaced the item (unless of course it is a dirty great big thing and she is seen on CCTV leaving the store with it). Chances are the store security have viewed it though and will be aware of her description and undoubtedly have circulated it to other local shops so next time she is likely to be caught.