If you can reach your silent alarm without this being noticed, do so. Otherwise, wait until the robber leaves. Have a prearranged signal with your employees that indicates a robbery is occurring. Do not use it if the robber can see you. Instead, wait until the robber leaves.
Thieves are less likely to steal if they think they are being watched.
Sales personnel can notice if merchandise is missing if it is displayed neatly in standard groups with three or four items per display. Place small, expensive items in secured display cases close to sales staff. When displaying clothes, turn every other hanger on the rack the opposite direction. When you apprehend a shoplifter, contact the police department regarding actions you should follow. Cooperate with law enforcement, and with the attorney prosecuting the case.