We are searching globally with a system administrator profile for a specific email and no results are shown. However, when using the "Mass Delete Lead" feature from the Setup, I can find a converted lead with that specific email.
How can converted lead be searchable for users?
Best Answer
Once converted, a lead record is no longer searchable in Salesforce Classic. However, the new account, contact, or opportunity record created from the converted lead is searchable. In Lightning Experience, both the converted lead record and the new record based on the converted lead are searchable; however, you can’t access the converted lead record to view or edit from the search results page.
https://help.salesforce.com/articleView?id=leads_notes.htm&type=0