yeah bud it could be cut or sprayed with absolutley anything, impossible to say what it could have been. Are you sure your nose wasn't just running and bleeding slightly, sometimes if your nose is running and theres a little bit of blood in it it can look kinda brown or yellow.
yeah its most likely just that gross yellow snot. I cant tell you not to do it because i do it 2-3 times a week(mostly on weekends). Its is not a good thing to use at all, but hanging out with some good friends, blowing some lines and having deep indebt conversations is my favorite part of the drug. Seems like everytime i do coke with friends i feel closer to them and usaully have great memories of that day. But i guess thats not a good excuse to do it lol.