Why did a man at work leave almost 6 months ago and I haven't heard from him since?

Left without warning. I also for some reason did not hear from any of his work buddies (such as our mutual work buddies) about him leaving or why he left and where he went. Which is weird. But I guess I am learning the life leason that work and personal life are actually totally separate even though guys will flirt and lead you on at work or even though anyone will act like your friend at work it doesn't mean once one of you leaves that you will even keep in touch.
Why did a man at work leave almost 6 months ago and I haven't heard from him since?
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