Salesforce searches a unique set of fields for each object.
Available in: both Salesforce Classic and Lightning Experience
The types of records you can search vary according to the edition you have.
Available in: All Editions except Database.com
When you search for a value in a field that's hidden from you by field-level security, your results include the record that contains the field. However, you can't see the field.
Searchable Fields by Object in Lightning Experience The records included in search results depend on whether the record’s object type and its fields are searchable. If you search for an object with a value that’s stored in a field that isn’t searchable, your desired object doesn’t appear in your search results.
Searchable Fields by Object in Salesforce Classic Each search type—sidebar, advanced, global, and lookup—searches a unique set of fields for each object. Your search results for a particular object depend on two factors: the type of search and the searchable fields for that object.