Are you talking about the progress bars in Mail?
I have always noticed that when I open the laptop cover after a period, when Mail connects with gmail, or iCloud or whatever mail service, I sometimes get the two progress bars that say that 50 or 87 or sometimes hundreds of emails are loading, even though I may have only two or three actual posts arrive and nothing shows up in the spam folder.
Always wondered what was going on, but since it happens on all machines and all Apple OSs, I never bothered to find out.
I guess you could call them ghost emails.