As someone who has an extensive background in security (including aviation security), the issue is not as simple as portrayed... one of the many issues is the sensitivity of the units being used - hence why some units will detect the belt and others will not. Additionally, not all belts have a metal composition which in turn complicates the situation moreso. Either way, the easiest thing to do is, if it has given you problems inthe past, remove the belt and continue on - it is quicker to proceed once than have to double back and attempt multiple re-entries.