Impurity on one's underwear

The other day I went to the bathroom to use the toilet and noticed a brown spot on the inside of my underwear. I am not sure how this had come as I had only passed stool the night before only, and after passing stool I had went for a shower/ghusl. In the afternoon around asr time whilst in the bathroom I noticed a brown mark on my underwear and found a mark only on the inside of the underwear. My pants was black so I could not see if there was a mark, I did check and found nothing. I had also checked the same spot on the outside of the underwear and it didn't look like any brown mark there but because it was material there was a slight smell. I had sat on many places during the day, some of these places were my car, rooms in an office, doctors office, home etc. Do these places become napaak wherever I sat? I am always very very careful with regards to istinja and as said I am not sure how this happen. It might have happened when I broke wind etc. Please advise urgently?