Sometime I get a "You have one new answer" message. But when I click on the envelope, there appears no new answers.
It's not that often, but every now and then I get this message.
Meta Stack Exchange is a question and answer site for meta-discussion of the Stack Exchange family of Q&A websites. It only takes a minute to sign up.Sign up to join this community
As Shog9 says here, it was an issue with new edits being grouped with new answers - this has been fixed.
Or you could have already seen the "new" answers. There's a delay between any new answers and the system flagging the new answer. One could have looked at the new answers and it would still be flagged later.
It might be an answer from yesterday (in SO time) that is not shown on the "recent" page anymore. Click on "yesterday" (or even earlier) to see it.
Edit: Now that I find this old answer of mine, I have to say that I have experienced similar quite often.