I finally found out what it means. It means the user is in private mode. If you see someone that's in private mode, try logging in a guest, and he won't be there. For instance, it will say "5 users are currently online", even though you only see 4 on the list.

"Stephen Wolfram is the creator of Mathematica and is widely regarded as the most important innovator in scientific and technical computing today." - Stephen Wolfram