Controlled, per ISO 9001:2008, 4.2.3, means that a document is approved by management before issue, it is reviewed and updated as necessary, changes to controlled documents and their most current version is identified, they are available when and where they are needed, and suitable identification is applied to obsolete controlled documents to prevent confusion. (Thus they are controlled by being designated as obsolete.)
ISO 9001:2008, 4.2.3: "Documents required by the quality management system SHALL be controlled." If personnel NEED a document, a sign, even a book, to perform operations affecting quality, technically, those documents are required by the QMS; any documents management uses to control operations affecting quality are documents needed by the QMS. Thus they are supposed to be controlled by management somehow. That is the principle. Often controlling these documents is viewed to be impractical, but the controls placed over such documents don't need to be cumbersome or difficult.
It's too bad when auditors stretch the principles of the standard to their most extreme, to a point where no value is added by the findings and discussions like this rage on. However, this does not mean we should abandon the principle. This means we should demand better quality from auditors.
Should anyone in a company be authorized to post a sign for everybody else to follow? Think about that before dismissing it, please. Or should such instructional documentation be left to management-type personnel? By approving a posted sign, it can be easily determined if the sign was approved and posted by management for all to follow. But a signature/initials aren't absolutely necessary. In the case of posted instructions, it's certainly one of the easiest ways to indicate approval.
Regarding documents of external origin, perhaps like the books you use everyday to do your work, the principle here is that these books, like your own procedures, may become obsolete over time. Without some control over these documents, there is a danger that bad documentation will be used someday to process work.
If a documented "reminder" or any document is needed to control processing affecting product quality, should these needed documents be controlled? By the standard, yes. If these documents are not needed, they can be removed.