My indicator.

       Dual purpose.  I suspect it was an Alice idea which dates back to the days when Front Doors were less draught-proof.  
       Indicator of what?  Firstly - In the past we have been known to find the Door unlocked in the morning = free entry to Burglar Bill had he been so inclined - AND - on one memorable occasion, so far in the past that I can't remember if pre- or post- Alice it was about 30˚ ajar.  NOT any more - BUT - no guarantee it'll be locked.
      Secondly - since I never alter the setting of the vertical Blinds the "Other two" can't tell if I'm "up and about".
SO - It indicates to me that the door's been locked; as it's not closed until the door is locked. AND - indicates, to them, that I've got up and opened it; either to collect Post or Milk OR at Lunch time if no reason B4.