Interesting research, Seden. According to some of these studies, the leached metal content of the soil can reach well over 1% metal.
At the southwest pacific coast in USA, people are warned not to eat muscles taken from the ocean tide pools because they concentrate heavy metals thought to originate from local effluent and chemicals that run off from storm drains. This makes me wonder: How much lead and other metals can earthworms concentrate that have been leaching into the soil at a military base? Are earthworms able to concentrate enough lead, cadmium, zinc and other metals to read on a metal detector?
In order to detect these metals that a worm might ingest, they usually need to be in the form of a solid chunk of metal. Finely ground grains and powders become very difficult to detect with metal detectors. Let us be generous and presume these worms were able to ingest enough metal to make up 15% of their body weight. If a ball of worms weighed 100 grams, then this would be 15 grams metal. If this metal is in colloidal form that can chelate into small deposits in the worm's body, and if these deposits are making good contact along the length of the worm's body, maybe there is an argument they "grew a thin metal wire inside them". Maybe a more likely argument is they swallowed some lead shotgun pellets and other metal grains that remained in their bodies in enough quantities to give a reading on a metal detector. In the worst case, Max found a ball of worms that swallowed 15 grams of gold flakes which they were not able to digest, and remained in their bodies. Then, being unable to determine why the worms gave a reading, he threw them out.
Best wishes,
J_P