My best guess is the person might be ninja AFK.
I am really guilty of this. I ran upstairs to get my son who woke up from his nap just today, and came back into my office three hours later.. only to realize I was logged in sitting at the EC tunnel the entire time. I think I had my AFK tag up (at least.)
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I know a few people who log into EC while tabbed out at work, so they can 'tab in' and check auctions.