Not one person in my gallery was asked. Candid really means open and relaxed. I often spend time around subjects, they know i'm shooting. But when i shoot them, they dont know. I am quick of the draw now, that comes with practice
This guy has no clue i took his photo, and i was using a 50mm was sat facing him
http://www.pbase.com/sean_mcr/image/56041858
Be he knew i was taking shots of random things, and got used to what i was doing. I waited to see if something interesting happened, and it did, he was engrossed in it, and was oblivious to me shooting him. It's timing, that's all it really is
If they notice you, smile. You're not dong anything wrong, and once they've relaxed and got used to somebody with the gear we have being around (trust me they always do) them, you then have a chance of getting some shots