Raise an automatic flag if a post that was undeleted by a moderator gets deleted by anybody other than a moderator.
If a moderator undeletes a post, this usually overrides a deletion made by the author, the community, or the Roomba bot. In either situation, deleting the post at a later date would probably be undesired behaviour and should therefore raise an alert.
A typical situation is this:
A user goes on a self-vandalising spree deleting questions and answers without clear reason (so no honest clean-up).
A moderator suspends the user and undoes the deletions.
The user does not delete their account (as such users often do).
The suspension expires.
The user deletes the posts again.
Sure, there are exceptions when the deletion is fine, but then the moderators can just not act on it. However, I would expect the false-positive rate of such a flag to be very low.
Note that the system completely prevents the opposite case: A post deleted by a moderator cannot be undeleted by anybody except another moderator.