I subscribe to what most people have said - a closed door indicates use. If the bathroom is vacant, the door is usually at least partially ajar - after all, most people don’t close it consciously after using it.
Heh. I have two small children, so I leave the door wide open. It’s easier than trying to open it from the john… They just love the potty! They insist on being able to see in at all times, especially if someone is “engaged!”
The 2 year old was watching me the other day, and when I finished, he applauded and said happily “Mommy went pee pee!” 
In my previous existence, I always closed the door, even if it was just me and my husband in the house. I was so modest, I didn’t even llike the dog to see in! But I have no modesty or dignity now. I have kids.