Be careful answering the door to those you don't know. Have you got one of those chains to stop the door opening fully? Always use if you don't know or are not expecting anyone. No professional such as a plumber, gas man or meter reader will not be offended if you ask for id and open the door with a chain on especially if a Female. In fact i think most would think that as the normal thing to do and would advise their own family to do.
The sign should stop any professional door to door sales people off.
