A friend of mine asked a question which caught me in surprise

he says
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hws it possible to find if someone is signed into gmail?
a friend of mine does it
with amazing accuracy
i sign into my account, but not into chat
he immdtly shoots a mail
"you are now logged into gmail"
n he's not the techie kind, how is doing this?
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Any clue what this could be?

another flaw on Gmail? (there used to be one like that few years ago, with mails titles)

Possible with Google apps Gmail.