Hello there Jared2w,
I understand you're curious your iPhone vibrating without a notification appearing, so check out the following information to see if it's helpful.
As you're not seeing a notification when your iPhone vibrates, are you aware if you may have received an email at that time? Depending on your notification preferences, alerts may be enabled; however, badges could be disabled. For assistance with checking this out, take a look at the following resource:
Change iPhone sounds and vibrations
If this doesn't seem to be related to what you're experiencing, Contact Apple Support to look into this in more detail.
Have a great day!