They probably do. Though, food industry metal detectors have to regularly be calibrated with check wands: plastic bars with very small embedded metal fragments designed to mimic possible contaminants like brush wire, oven conveyor pieces, etc.
When I was QA at a food plant that used them, there were numerous problems with the metal detectors.
- Line operators were haphazardly trained on calibration and how fucking important it is to do it correctly and on time. Lots of operators rushed or skipped calibration and falsified the logs. The fact they were rushed, overworked, and underpaid didn’t help.
- The detectors had a high false-positive rate, leading to a “oh it’s probably nothing” attitude.
- Management didn’t want to spend ANY money on maintenance and upgrades that would have prevented metal contamination or decreased detection errors.
To this day I still have a lot of anxiety eating factory food because of the nightmares I witnessed at that place…
Could be one batch or many. A “batch” means different things in different food industry contexts and has more to do with ingredient tracking than production timespan