Years ago, as I was sitting and drawing a woman who looked a little, let's say, mad (like crazy), she suddenly jumped up from her seat and sat down next to me. I immediately shut my sketchbook. She leaned over my book and asked what I was drawing. I reopened the book at random, relieved she hadn't seen the drawing I'd done of her. Since then, I try to be discreet and to not look too obvious; I don't want someone to get upset with me.
To draw people in a closed environment like a bus, I glance at people once in a while and try to remember details of their faces, adding them in afterward if necessary.