The simple solution is implemented as follows.
Open Drive audit
Apply custom filter and search for Shared out files which were updated more than 3 months ago (In this case we just choose a date which is 3 months in the past).
Use the example above (just adjust the Updated date), then hit ‘Apply’.
Once the search is complete you will see the result with all the Shared out files which have not been updated in the past 3 months.
You can quickly see how many files overall there are from the result received. Simply select ‘Show stats for the current filter’
Note: If you want to ignore – Trashed files (see from the results above), simply add extra search field and ignore them. You can exclude the “deleted” too.
This shows all files not read in the last 90 days that are shared out. This may be quite a large number of files.
Using ‘Schedule/Save’ you can do the following.
Run a daily or monthly report to get this list.
Automatically revoke the external sharing. (using Scheduled reports under)
Warn the local owners with a tailored message asking them to remove the share.
Or you can do any combination of the above.
You can also run a one off job to do the same.
If you schedule this task to run daily, the 90 day lookback window will automatically be moved forward on a daily basis.