At first I was going to say, “They might have opened it to try to figure out who it belonged to so they could return it, but they should have stopped reading the moment they realized what it was.”
But then I got to this part:
Completely inexcusable. You have every right to be furious.
The OP stated that they mistakenly left the journal as work, which doesn’t strike me as unprofessional. Given the content which may be there, it would be something which one should be very careful about, of course.
Omg! My coworkers go into my backpack and read my diary and I wrote about being assault I am extremely pissed and embarrassed and I have no way to prove it except by the things they say.
Of course they go fired - workers knowing each others’ salaries hurts management and helps the workers. The last thing management wants is workers asking why someone else makes more than that do, and why management makes SO MUCH more than they do.
And under most union contracts, knowing every non-management employee’s pay is common knowledge. All you need to know is their job classification and hire date and you can probably figure it out quite easily.