Final answer:
When creating a new user in Salesforce and encountering a message that the username is already in use, it is likely because Salesforce requires usernames to be globally unique across its entire platform. Usernames are not case-sensitive and the issue is not related to the recycle bin, deactivation, or account holds within your org.
Step-by-step explanation:
When creating a new user in Salesforce and the system reports that the username is already in use despite not finding it in the organization, a possible cause is that Salesforce usernames must be globally unique across all Salesforce instances. Even if the username is not found in the current org, it could be in use elsewhere. Furthermore, Salesforce usernames are not case-sensitive, meaning that 'User example.com' is considered the same as 'user example.com'. Also, Salesforce does not place usernames into a recycle bin, and they are not affected by being deactivated or on hold within your org.
Usually, in this situation, you would need to choose a different username. An alternative is to contact the individual who has that username or Salesforce support to see if the username is indeed active and being used; if not, they may be able to assist in freeing up the username.