Random Apple mail *new mail* sound with no mail?

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I have ghosts in my mac.

For some time, I have heard what sounded like the Apple mail *bing* of new emails coming in when there was no mail.
At first I thought I was hearing things or mistaken (I was always in another room and either playing music or doing other noisy stuff ) but a few days ago, I was actually sitting in front of my mac, in silence, browsing my phone and clear as day, the mac played its "new mail" sound while there was no mail.

No "normal" mail, no mail in the junk folder, in any rules and the last real mail arrived more than ten minutes ago.
What gives?

 

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I seem to be having the same issue. I made a new post as I am not sure it's exactly that. But I have turned all my mail notifications off and it still makes that downs randomly and with no notification showing up in the center. I keep my volume down but then I miss other stuff.