Security Warnings

There are lots of reasons you might see a Security Warning in the message bar when you open a workbook.
The two most common reasons are to tell you there are external links or there are macros.

Multiple Issues

When more than one security alerts needs to be displayed, then you will see the following prompt:

(.xlsm + webservice function)
enable content - dialog "Do you want to make this file a Trusted Document"
Check your Trust Center - Trusted Documents

External Links

This message appears when a workbook contains at least one external link.
Check your Excel Options > Advanced > General > Ask to update automatic links
Check your Trust Center > External Content > Prompt user on automatic update of Workbook Links