Why the number in the system is wrong
Stock records do not drift because people are careless. They drift because a dozen ordinary events each move stock without moving the record.
When labour is part of the cost, a short period of task-level recording is more useful than estimates. For a workplace example of structured time capture, see this page.
For a broader reference on product and location identification, GS1 standards document widely used supply-chain identification standards.
For general measurement and standards context, the National Institute of Standards and Technology publishes public technical references and standards resources.
Read the guide