I accidentally noted that a particular question does not appear on the list of questions when I am not signed in. The question comes up in the top five once I am logged in. Why is this so?
Is it a bug in the feature?
As Charles notes, the list of questions on the homepage is altered for anonymous users: instead of listing recently-modified posts, it attempts to pick questions that may provide a good example of what the site's about.
Currently, that means it's very similar to the monthly hot questions list.
If you want to view a list of recently-modified questions without logging in, you can find it in the active questions list.
The ordering of questions when unlogged seems to be quite different from that observed when logged in. I can observe the behaviour you're mentioning with other questions, so it doesn't seem to be specific to that question.