[Friends online, difference between website and inworld]

l will do a more scientific study with an alt, but I know that if somebody who has also friended my alt decides to hide, it doesn't matter which account I look under - that person, when hiding, is simply not on the list. When that person is not hiding they will show up on both lists. It's actually convenient, given SL's poor implementation of hiding.

[Friends online, difference between website and inworld]

I've looked at this carefully, with one avatar I know is prone to "hiding" their online status. The Dashboard friends list (left side column), lag issues aside, will tell you if your friend is online, offline, or possibly hiding. If they show as "offline" on your friends list inworld, and "offline" in your web dashboard, then they are simply "offline."  However, if they show as "offline" inworld, and do not appear at all in your Dashboard, then they are hiding their status.

Personally, I look forward to the day when we can simply appear offline in all lists, groups, "IM for status" attempts, Greenlife, etc.  There are times I just do not want to be disturbed, during work, or play, or just because I want a quiet hour or two on the grid. I've actually lost newish friends that way because they thought it was personal.

Anyway, that's what's going on. The list is a dead giveaway if somebody is hiding.