And for once, @jaquo, I am going to disagree with you. I share my home with my guests and there are times that I need to know if they are in or out. Every day I need to gather their dishes and coffee canister, grind fresh coffee, and play Breakfast Fairy. I prefer to do this while the guests are out.
And then, in the evenings, I leave the door unlocked for the guests’ return. If they are already home when I go to bed, I lock the front door and turn off the lights. If they are out, the door is unlocked and lights are on. I leave them a note to remind them to lock and turn off any lights that they can find.
And finally, my guests have a suite of rooms on the third floor. In the event of a fire [a real possibility with older housing stock], I have no interest in sending firepeople to the third floor to save people who are not there.
All of that said, I have no fool-proof system to know when people are in or out. If they come home when I am not there, I can see that they have used the key. If they prefer to take off their shoes in the foyer, I can see that. But then there are the “silent” ones. I simply don’t hear them go up the stairs and then they are so quiet while in their rooms, I don’t hear them then either.