At a place I used to work they did random drug testing, and the only time I saw it happen was right at the beginning of a new year, my boss had one guy tested:

At a place I used to work they did random drug testing, and the only time I saw it happen was right at the beginning of a new year, my boss had one guy tested: the 100% straight-laced Asian Christian teetotaler. Maybe it was actually totally random (I highly doubt any of us used any kind of drugs), but I always figured his boss said, "you have to randomly drug test someone", and he didn't want to lose any of us so he picked the sure thing.