Yesterday I received some Citrisurf 2310 for passivating and a copper sulfate test kit. The test kit is a liquid that changes color in the presence of free iron.
I ran the test kit on two vessels - one from the factory that i hadn't touched and another that had been used, rusted, and that i cleaned up with oxyclean and then a detailed application of bar keeper's friend before letting it air dry for a week.
The bad news is that the unused and uncleaned vessel straight from the factory failed the copper sulfate test -- specifically along the welds in the upper collar. The good news is that the used vessel that had been cleaned and "passivated" with bar keeper's friend and left to air dry passed the copper sulfate test.
I went on to truly passivate the failed vessel with the citrisurf 2310 product. It passes the test fine now.
The citrisurf is a really nice product and is probably something that anyone who regularly uses stainless in their brewery should have on hand. It's easy to apply, non-toxic, and noticeably changes the stainless.
Going back to the Vessels -- if I had to guess, I'd say the tools in the factory had bits of iron hitching a ride. The good news is that it seems it can be cleaned off relatively easily.