J
Personal notes should be prohibited. The risk of error is too big, controlling personal notes is a waste of time, information is lost and worse, improvement is blocked.
It is also important for the purpose of THIS THREAD that the OP has not given any indication the official work instruction is not followed, merely that there are operator notes about the work instruction present at the work station.
Wes again:
I fear many auditors fall into the trap of imposing their own sense of right and wrong on a process they observe. Throughout this thread most experienced folks have agreed there is no "shall" presented by the OP which has been breached.
We might all agree the management of the OP's target organization "should" be proactive and address the issue...
...either making private notes acceptable or banning them, BUT they "ought" to be able to provide cogent reasoning, communicated to all pertinent staff, for whichever decision they make, not merely present it as an administrative fiat akin to forcing men with shaved heads to wear hairnets because "some" people with long hair may get it caught in machinery.
And see what effect that has on the men (or women) with shaved heads ... and I assure you, it won't be positive.
We should be expected to understand risks and risk mitigation, and that process shouldn't include knee-jerk reactions. If there is a significant risk of something bad happening, the appropriate measures should be invoked, but risk should be evaluated on a case-by-case basis and not by a Pavlovian response.
Nor this:
Issuing blanket edicts (when there's no substantial evidence that an issue is widespread or likely to have untoward consequences) is a path to fear and loathing.
) Healthy disagreement is just that - healthy! 
Where