Create a Task Report that shows a column for the (multiple) Assignees, the Primary Assignee, and the Last Updated By user. The latter will often (but not always) be the same person who marked the Task as complete, so that person is not the Primary Assignee or among the Other Assignees (which, with some extra effort, are inequalities for which you could filter to retrieve only such exceptions) will then show you such Tasks of Interest (insert sound of wrist being slapped)
Create a NOTES report to retrieve (and perhaps filter; likely considerably harder....) the full audit of all Tasks closed within a certain (prompted) timeframe by a certain (prompted) user to confirm "they ought to have done so"
And my personal favorite (and recommendation, if it's worth your time), I invite you to adapt my Targeted Auditing Proof Of Concept to track EXACTLY the time and user who set a Task to complete, right on each Task, even if it happens multiple times
Sorry to be digging up this very old discussion, but I'm having the same issue and found your reference to this "Targeted Auditing Proof of Concept"...of course the link doesn't work anymore. Would love to check it out if it's still online somewhere!
