Sketches of people who don't know I'm sketching them

I'm glad I put my A6 Moleskine into my bag again.  Today it prompted me to do these 2 little sketches, the first while I was waiting at Specsavers and the second while I was on the #45 bus going home.

I enjoyed doing these in the same way that I enjoyed the cat sketches.  I like drawing fast. I tend to find that if I spend more than a minute or so on composition the whole drawing just goes wrong. I've done loads of life drawing classes and more often then not really disliked what I've produced from them. I like the type of life drawing where the person running the session gives you just 1 minute to get the pose down but when I'm faced with 10, 20 or heaven forbid 60 minutes drawing someone I just get really fed up with it all.  Another challenge I find is that although I don't have any problem with drawing in public, I do get a bit hesitant about drawing people in case they turn around and see me drawing them. I don't know why it concerns me. I'm sure people would be flattered if they knew they were being drawn, I know I would.  And I'm never sure how obvious it is to someone that they are being drawn.  Do I look like I am sketching faces in my Moleskine or do people just assume I'm writing a shopping list?  I don't have the same hesitancy if I draw people in coffee shops though so I wonder if it is to do with the type of people I'm drawing. Do I unconsciously assume that students and business people in Caffe Nero are more open to arty stuff than people on the bus?