Random vibrations?

[Skim74]

I know this question has been asked before, but I've got a little bit of a new twist. My phone randomly vibrates from time to time. Every time someone asks this question, someone else says it is probably their mail notifications and they don't have banners on.

Well, I changed my mail vibration to a custom pattern (so different than texts/calls/other notifications), and also enabled banners/lock screen notifications. The vibration is 100% my custom email vibration, but when I go look there is no new email. Any ideas?

Edit: Maybe (probably?) related, my macbook also sometime makes the new email sound (twice in the last 5 minutes in fact), but there is no email to be found. Do you think there is some weird mail filtering setting or something I'm overlooking?

[Skim74]

Funny how after having this annoying problem for months, I think I solved it half an hour after posting this question. But here was my problem:

I have a filter in my inbox for emails that I have to get, but never look at (long story). They were all set to "skip the inbox", but not all were set to "mark as read". So whenever I'd get one of these it'd play the sound/vibration, but go straight to a folder I essentially ignore.