the 2 hour window that your HACCP team has selected for verification, is based on the Hazard Analysis.
If this is the case, I agree with the above, rework according to your HACCP plan, if you plan says it, you have to do it. That's why everyone signs off on it. :) If they want to change it for the future do it as a preventive action.
HOWEVER, monitoring intervals aren't always based on the hazard analysis, they can also be based on scope of rework. For example, I have a check on our production line that relies on chemical dosing, and I have high confidence (based on years of data) that it rarely goes out of spec and needs little adjustment with an experienced operator.
Based on that hazard analysis, I potentially only need to monitor the status at startup, after breaks, and shutdown. However my only corrective action available if that check fails is to rework all product since last good check. So while I have confidence in the equipment, we instead check it every hour so that in the event of a failure we only need to rework one hour of product instead of 4-6 hours.
My corrective actions in my HACCP plan reflect this, so while the actual monitoring in the plan is stated for startup and shutdown, the daily production paperwork has my guys checking it every hour to minimize the scope of rework in the event of a failure.
If you feel good about your metal detector, you can revisit your HACCP plan and do the same thing without changing your practices on the line. However if your plan doesn't say it now, you're stuck with the plan as written, otherwise what's the point of the plan? Say what you do, do what you say.
Metal detection checks are often driven by customer requirements however, rather than your particular hazard analysis, so you need to make sure you include those in your assessment.