Clients:Log Tab
From WHMCS Documentation
The Log tab lists entries from the logs under Activity Logs at Configuration () > System Logs or, prior to WHMCS 8.0, Utilities > Logs. This provides a detailed audit trail of the client's history within WHMCS.
You can access this tab when you view a client's profile at Clients > View/Search Clients.
Search
You can search logs by the date, username, description, or logged IP address.
To do this, enter the desired search criteria and then click Filter Log in the top right corner.