Every time I mention a low-tech security device called a busybody, I confuse people. We have them in Philadelphia, where they are alleged to have been invented by Benjamin Franklin.
A busybody is an arrangement of three mirrors held by a bracket. The bracket is attached to the wall near a window, usually on an upper floor. By adjusting the mirrors, you can see who is below without making yourself visible. Usually, you arrange the mirrors so that you can see who is at your front door, so you can decide if you want to go downstairs and answer the door.
Here's a link to a Philadelphia-area company that sells them. (No, I don't own a piece of this company. But it's annoying to search for them on the web: you keep coming up with the more popular meaning of the word.)