Missing contacts in Apple Mail

On a new MacBook Air, running OS10.13:

Helping a coworker track down a missing/not sent email I ran into a strange problem. When she typed in a name, it pulled up three unrelated email addresses. For example: "Bob *****" had attached to "s*****@ mail.com", "f*****@ compuserve.com", and "d*****@ aol.com"

Simple, I thought. I opened her Contacts app to edit this but it only contained 3 contacts: Apple, her own, and one other random person. Yet when she is doing email, it is auto completing the names she types and adding the addresses!!! (Though sometimes the wrong address.)

So...

1. Where could it be getting addresses from if not from Contacts?

2. How can I find and edit the records?

Very odd!

Thanks for helping!

[Personal Information Edited by Host]