The search field should be a global search and organize the search results by type of data. It would be nice to enter a value, and know that search is looking through all major data points to find the term. As of now, the default is for the search to look at Customers. If you want to search Invoices, you need to first select invoices, or set the search parameter for the search to Invoices.

As a reference, your own Zoho CRM, and the competitive Quickbooks both function with global search.