There is a scenario where the Inventory Quantity is already 0, but the value is still there.
One of the cause is because of the Charge (Item).
Post Purchase Receipt with the cost of 1000.
Post Sales Shipment.
Post Purchase Invoice, Charge Item with the cost of 200 , assign it to the Purchase Receipt.
Run the Adjust Cost Item Entries.
Run the Inventory Valuation report.
It will show you on the Actual Cost line, that the Qty. is 0, but the Value is 200.