For a long time, I have been using store bought baking soda (with sodium bicarbonate as the sole ingredient listed) as the sodium bicarbonate in my (qualitative) home experiments, but recently I noticed that a 100mL solution with 3.6g baking soda developed light-yellow precipitates after air exposure for two days. I was pretty sure the baking soda had all dissolved initially.
Also, I started to worry about this because of the weird smell associated with baking soda when heated.
Anyways, what are the main impurities in baking soda? I especially want to know about the source of the yellow precipitate and smell, but a full list or some articles on this topic would be most appreciated.

Searching in search engines and online databases using terms like "baking soda smell" gives results about using baking soda as a deodorant; "impurity in baking soda" does not yield an answer; "baking soda composition" only shows sodium bicarbonate. The vendors I asked persisted that their product contained nothing but sodium bicarbonate, "100% pure with no other substance." (It's unlikely that they admit the presence of impurities, as they think I would refuse to buy "impure" baking soda!)