Yes, Inventor documents must be opened before we can access their iProperties. I am not really sure how they support the ability to right-click on Inventor files in the Windows file explorer, choose Properties in the context menu, and be able to get access to some of the iProperties that way. Must be an external helper app that gets installed with Inventor, or the Inventor SDK stuff. As I mentioned though, files can be opened invisibly, so that there is no document tab for them, and no 'view' for them in the user interface. Opening them that way makes stuff process faster, because there are no 'graphics' to process or keep updated.
This is sounding like it might potentially be either too difficult, or too long running for a simple iLogic rule, ran from within Inventor, to handle, but I'm not sure. Sounds like it might be better suited for 'Apprentice' which is sort of like a lighter version of Inventor, with no user interface, and very limited abilities, used from external EXE type applications, instead of Inventor, for doing lighter tasks like reading or editing iProperties, inspecting stuff, and minor tweaks. I have never used it before myself, because I can not develop EXE's right now where I work, due to corporate security restrictions. It sort of depends on how many files there are, and if it may run into any files that have 'issues' that might pop-up when opened, and such things.
You still did not answer my question about what should happen when a file is found with matching text in its Comments iProperty value.
I do not have Vault, and have never used it myself, but yes, from the sounds of it, that resource would likely be very helpful in a situation like this.
Actually, now that I think of it, if you do not have Vault, you could try using the Inventor Design Assistant application. It is a separate application that usually get installed when you install Inventor. It has various uses, but I think one of its uses is for 'finding Inventor files' that match certain criteria. And I believe it will let you search within their iProperty values also. It should let you specify what types of Inventor files, and what project area or directory to search within also.
Wesley Crihfield

(Not an Autodesk Employee)