Jerry, it is also necessary to remember that Thiocyanate will only release CN gas with acid and lots of heat. The same is true of Ferri and Ferro Cyanides. Usually the biggest concern with these two ions is the COD (Chemical Oxygen Demand) which uses up oxygen in water and kills wildlife and the BOD (Biological Oxygen Demand), which is the Oxygen needed to break down the chemical by organisms in the water.
Both Thiocyanate and Ferri or Ferro cyanides are controlled more for their BOD and COD than the formation of Cyanide itself.
We went through this all with the government when converting from Ferricyanide to Ferric EDTA and then the possible use of Thiocyanate in Blixes. School Finishers of Webster NY was shut down for excess Ferricyanide in Irodequoit Creek and thus became the first trade trial of Ektaprint 3 chemistry to avoid the Ferricyanide COD and potential COD. We removed the Thiocyanate from the blix for the same reason.