Thanks that's interesting. I've seen this exact thing happen with stainless steel eating utensils (commonly called "silverware" in the US for historical reasons despite being stainless steel). In a storage unit, I found some very rusty spoons and forks that were in a box with other rusty things. The conclusion I left with was "stainless steel can indeed rust" but I didn't know why, so this is nice to know!
If you really want to blow your mind, your "porcelain" bathtub can also rust.
Learned this the hard way when a cat started pissing in the tub. Between that and using bleach for cleaning, the enamel was eroded down to the metal casting. Then, rust.
Advice online will say no, that red shit is calcium and mineral buildup. That mostly happens in toilets. Unless you bought a tub in plastic or fiberglass, it may well be actual rust, especially if it reoccurs very quickly. Treat it accordingly before it drains through your floor.
