An interesting question, no? I must admit, I have quickly browsed an open diary, many years ago. I probably wouldn’t now, unless reading it can help me to help the owner or further understand some of their life choices.
I would have to say I would close it and ignore it . I would close it to keep other eyes from being tempted to read it I would ignore it because it isn’t my business.
I will just leave the diary open, would not even read it, or simply just ignore it.