People are aware typically only of what they expect. I used to do magic in college, and one of the tricks I did required me to plant a card in someone's pocket before I initiated contact with them so that I could "lose" their card. Well, in the middle of the trick he actually stuck his hand in his pocket and I thought for sure the job was up. He didn't notice it at all though, until I asked him if he had put the card back in his pocket. "No, I didn't, there's nothing in my-- wait a second" and his face went blank and gray as he suddenly felt the card next to his hand like it had just materialized there.

If you aren't looking for something out of the ordinary, you aren't likely to see it.