If your house is for sale and you see someone walking around your house looking at it, do not presume that they are interested in buying it. They may even knock on the door "to get a better look".
Never let these people in your home. Instead, either give them your real estate broker's contact info or just point to the 'for sale' sign.
There are all sorts of reasons why someone would want to see the inside of your house. Many of them have nothing to do with having a genuine interest in buying your property.
I am surprised by how having a house for sale (with or without a brokerage 'for sale' sign) emboldens some people to think that it is a license to take liberties that they would not normally take with homes that are not for sale.