He strays from his predictable everyday behavior.
He has a rapid change in weight or appearance.
He has unpredictable mood swings.
He remembers things wrong or loses track of the details.
He has an overnight change in musical taste.
He suddenly needs a lot of privacy.
He hides his financial history from you.
He has a drastic change in sexual appetite and style.
He’s secretive with his technology.
He has new “friends” at work and/or more work responsibilities.
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From @parag