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I am Pete, and I'll be happy to assist with your question.
This is a common problem with shared documents accessed over networks - it is not always due to a guilty person still having the document open.
Anything that accesses files in real time (antivirus, syncing, backups or indexing software) can also cause this message to appear.
If it is not due to a guilty person then why does it happen, and what can be done? Does my small office need to call in an expert to correct the issue? Is it a serious issue?