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?

I think you should try calling home periodically through out the day and see if she answers the phone. If she is sneaking out she won't be at home to answer it in the first place. Try to confront her about it if she doesn't answer the phone.