Our Dopper Insulated is produced from high quality 18/8 stainless steel. A fantastic material, but also requires some care. Stainless steel has a high resistance to rusting, but it does not offer 100% protection. So it is possible that you still have a dot of rust.
If dirt is not rinsed off, or the bottle is not dried sufficiently, iron - naturally present in water - can react with oxygen in the air. This leads to rust, especially in the rounding (just below the thread or in the transition with the coating). Standing water is the enemy of any kind of steel.
It is therefore important to thoroughly clean and dry the bottle after use.