The Document Inspector is a relatively obscure Office feature which helps identify elements of documents that could pose a security risk. The next updates for Excel, Word and PowerPoint add some new options to check for, including checking embedded documents..
There are plenty of ways documents can reveal unwanted information, from revision history to personal contact details. The beefed-up document inspector, which is being added to Office 2013 and 2010, expands its scope to also check for embedded documents (which will be accessible if you share the document, even though they might not be fully visible). It also adds support for checking macros, and several other features in Excel.
You can access the Document Inspector from the main File menu under Info. For maximum security, it still pays to save your document in a non-editable format, but this is definitely a helpful update.