If the file used to have macros and they were removed, it can still get incorrectly flagged by the security features as it can see the module exists even with no code in it.
Steps to reproduce problem:
Open Word document.
Open VB IDE (ALT-F11)
Find the document project in the navigation pane.
Right click > insert module. This creates a branch for "Modules" containing "Module1".
Don't add anything to Module1.
(This is basically the same as if someone recorded a macro then went and deleted it but did not actually remove the module).
So, for your troublesome document, open it, then ENABLE the content (if you don't you won't see it in the IDE).
Alt-F11, go find the (empty) module and right click > remove. Don't bother to export before doing this (since it's empty, right?)
Save and close the document.
If you are unsure about the provenance of the document and don't feel happy enabling the content, this should work (your mileage may vary):
Open doc. Don't enable content
Find the document and right click > insert a module.
Now remove that module and the IDE deletes the modules container with it, along with the disabled modules.
Save and close