Your individual magnets should be identifiable by some method, and recording which magnet # and the location it is installed is helpful information. I worked in a plant where we had more lines than magnets, so there was cross use we had to capture. Something in your sheet to help you know magnet #4 is being checked on line 2. Also helps to document when/if a magnet ever gets removed from service or a new one is introduced, as your paper trails will show the magnet no longer being used after you document it's retirement.
In my old spice plant, we would get metal powder/dust or small fragments from the product pretty often. During the magnet check, operators would report findings as "light, medium or heavy" for that check, and we defined directly on the form in the top instructions what each meant ("light" would be basically a dusting of metalic powder, "medium" might include up to x amount of metal fragments under 2mm or magnet mostly covered in powder, "heavy" would be any larger fragment, something like that). And any large item found was required to be described on the bottom in the notes: "2pm magnet check discovered a 10mm bolt" or something like that. Auditors almost always commented that they liked our definition of light/medium/heavy being shown directly on the check form, as it prevents interpretation by different operators.