Imagine you're a guest in someone else's house. While checking your email on their computer, you discover they are logged into their LJ. What do you do?
a) stop reading as soon as you realise, because, well, eewwww.
b) have a quick peek, what harm can it do? But keep your mouth shut.
c) carefully read through messages on private filters you aren't on, and then check the comments to work out exactly who else is on that filter.
d) round on the author of the filtered posts, demanding to know why you weren't included in the filter and why other people were.