There’s two aspect to using fields in Search within CRM. The settings are in separate places, so I’m just making this mental note to check both when making configurations.
Field properties – in the field’s properties, you can configure a field to be searchable.
What this does in fact is it allows the field to be indexed for Advanced Find only.
The other aspect is if you want a field to be indexed for quick find search. You do this by going to the Entity, navigate to Views, select the Quick Find view and add a Find Column.
In here you can add Find Columns, and you can also add the columns to the view, so you know why that specific result is returned.