Background (pointform for readability):
I prepack oral medication for a hospital.
Our work area consists of a long stainless steel countertop.
We use plastic bins to transport the medication around the pharmacy. To clean the bins, we use cloths premoistened in a 0.3-0.5% solution of Hydrogen Peroxide (can't remember the exact concentration).
While the bins are drying, they're stacked on top of the counter. The bins at the bottom are often left facedown, which prevents then from drying properly.
We noticed bin-shaped stains appearing on the counter where we let the bins dry. I figured the Peroxide was oxidizing the counter.
70% alcohol, water and goo-b-gone had no effect on the stain.
Today, something was allowed to dry on the counter. It dried to a white powder. I'm almost certain it was a 4 mmol/ml solution of NaCl, but I don't know for sure. When I cleaned the counter off, the part underneath the power was completely clean! It looked brand new. We tried applying more NaCl to it, but we didn't have time to let it dry, and there was no noticeable change when we cleaned it off.
What I know/don't know:
I'm pretty sure the counter is stainless steel. We have signs up in the area with directions for cleaning stainless steel countertops.
The solution was almost definitely sodium chloride. If not, the only other thing I can think that it could have been is Sodium Phosphate; but I've never seen it dried, so I don't know for sure.
What could have happened here? I know rust can't be "undone". The only thing I could think of is somehow the top layer was removed; but I can't see saline doing that.
Can anyone think of what may have happened?