There is the dog and, as Stacey has said, if you have an ethical, reliable handler that the dog doesn't react to or take cues from, then the dogs are considered more reliable than the ion scanner. There is the X-ray machine that you put all of your belongings through, your purse, coat, everything; you take your shoes off, and your shoes go through. You go through the metal detector.
There are those three things, and then when you get into the visiting room, there are cameras everywhere for observation by the correctional officers in the bubble. You're under direct observation and prone to audio on the tables, so they listen to conversations. All of these security measures are in place, and if you leave to go to the bathroom, you're searched. You're patted down on the way back.
All of those are already in place without the ion scanner. The only time I've seen a visitor caught for having brought drugs in, it was from the cameras and direct observation by the correctional officers.