I came across a line in my Nginx access log:
18.104.22.168 - admin [12/Dec/2012:18:33:18 +0800] "GET /manager/html HTTP/1.1" 444 0 "-" "-"
Let me stress that there is only 1 record with this IP.
Notice the authenticated user admin.
After some googling, I was able to find out only that this is authenticated user (http://wiki.nginx.org/HttpCoreModule#.24remote_user), which was authenticated by the Auth Basic Module (http://wiki.nginx.org/HttpAuthBasicModule).
However, nowhere in my site (configuration) do I use HTTP basic authentication.
What is going on? How did it get there? Was the user authenticated?