Naturally Occurring Sulfites or Sulphites form In the following link, it says that the product 


  May contain trace amounts of naturally occurring sulfite residue


I asked the manufacturer about it and the explanation I got was:


  The trace amounts of naturally occurring sulfites are present in hydrolyzed collagen (as are most other proteins as well). The sulfur that holds the bonds between the molecules is broken up from these processes and changes them to sulfite


Does anybody knows the exact chemical form of the sulfite that this product contain?

Is it in the form of an anion or in the form of a compound?
When collagen is hydrolyzed, sulfite ions ($\ce{SO3^2-}$) are released. Usually manufacturers filter out the residues, but may some still remain behind, which is why they put the warning on the label.