Acid pickle contamination

My company supplies hydrochloric acid to the electroplating industry and recently a customer complained that our hydrochloric acid was contaminated .We deny his claim.
The customer said that after adding 100 lts of hydrochloric acid out of a drum into his pickling tank the pickle turned a light grey in colour and emitted a pungent odor, a strong smell of sulfur. It is a barrel zinc plating line.
The parts he was pickling at the time were steel components that turned a dark grey colour.Our tests showed that there was a small percentage of sulphate in our hydrochloric , which the customer says it was the cause , we are saying it is negligible .
The strength remained at 33% and s.g was 1.14.
We believe hydrochloric would not contaminate a pickle tank.
The acid pickle already had an inhibitor in it , so we believe that is not a cause.
We are saying the reason for the contamination in his acid pickle was the steel components themselves, what was coated on them, and what was dissolved in the pickle.
Has anybody heard of this happening anywhere before? , is there any cases that have been documented anywhere?. Where did the sulfur odour come from?.

Pickling sulfur bearing steel such as 12L14 can cause an odor, however it is not usually strong enough to be a problem. In any event a HCl pickle should have exhaust ventilation.