Also it depends with the owner.Although I have not come across such situation. The question itself does not really specify what that someone is. If the diary is for my teenage child I would probably have a peep. Teenagers are secretive with their diaries so if they leave it open most probaly it would be deliberate, they want you to see it. It could be that, they want to communicate something sensitive to you and they don’t feel confident approaching you in person. They could be being bullied or struggling with their mental health.