As a ghost, of course you can't be seen by anyone.

You're wandering around a street until you saw a big mansion. You decided to go inside. There was many people there. So, you go inside a room. The room was a big bedroom with many flowers plus a open terrace. Suddenly, someone goes inside the room and saw you. You thought she couldn't see you. So, you just stood there. She looks frightened.

"Aah! What are you doing in my room?!"