OK.  Now we just finished (for the moment) talking about your front door.  It assumed that you had a reception area separate from your personal office.  But what about the front door to your office?  It has to be open during office hours right?  Usually that’s correct. But you should have a bell/buzzer that shows when the front door opens. They are cheap too.  Just a little ringer mounted high up so the door touches them as it is opened is enough.  You just want something so you know somebody’s come in.  If you have an assistant, she (or he) will want to know too.  Nobody likes being surprised by someone standing behind them, even it is is innocent.