Impurity on one's underwear

Q: 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?

Hearing one's own recitation in Salaah

Q: Question regarding reciting inaudibly. In my case I was praying Dhuhr Fardh and quietly as to hear my own voice, in my last (4th rak'ah) - I was reciting "Ghayril Maghdoubi" at Maghdoobi, I could only clearly hear "Ma--doobi" the Gha wasn't proper so that I could hear, but I moved my tongue to recite it as normal. Obviously I didn't want to recite "Ma--doobi", so I think I inaudibly said the gha, does this matter or affect the validity of the namaz?

Wearing makeup

Q: I have a non muslim friend who was asking me that muslims are not allowed to wear make up then why do you wear it? I said to her that I only
wear foundation to hide my dark circles and it looks natural so I'm not attracting. Is it still not allowed?