This can be to do with allocations that need to be allocated for example a receipt against a bill, it will be matters that don't have an office balance.
A report can be run off with just the zero balances, follow the below steps:
Select Advanced Report Execution.
In the Accounts Standard/House Keeping (if PML level), select Client at Practice Level/House Keeping (if Client level) folder.
Select the report called Matters with Zero Office and Outstanding Balances.
Select RUN REPORT.