We do it in private - hopefully. When we think no one can see us. However, many times, if we are stuck in traffic, we don't think anyone else is looking through our windows, and we mine for gold.
We do it at restaurants - when our date leaves to go to the ladies' room - probably to pick her nose - and we uncover the lost boogar of doom.
We do it in bed. When we think everyone else is asleep and once we get the goods, we flick them across the room and judge how silent the night is based on how loudly we can hear the landing (and of course we tell ourselves we'll vacuum in the morning).
But can anyone actually say they never pick their nose?