I think my helper has been sneaking out in the daytime while we are away at work. However I cannot confirm as I do not have a CCTV at home. What should I do?

Talk to her first. Maybe there's a valid explanation for her sneaking out: some pressing family or personal errand. Benefit of the doubt. If this persists, call the house during the day then if she doesn't answer, ask her where she was once you get home. If you feel like she's being dishonest, tell her. Then, give her time to correct her behavior.