The kind of honesty when it comes to helmets on bikes really amuses me. An acquaintance once told me that he has the habit of leaving his helmet on his bike unlocked and it has been safe always. And he told me that in past year he has not had his helmet stolen even once.
That piqued my interest and I decided to try it out myself. No matter where I park my bike, movie theatres, office, temples, shops... I conciously make it a point to place it on the bike's handle-bar. Mind you, my helmet is relatively new (Just a year or so old). I've been doing this for over a month now and should agree with my friend. It's been totally safe. Now, touchwood.