The earlier in the day you go, the more likely you are to be picked. That's what we noticed on our last trip. Before MK opening hours - every adult got sent through. When it got busier - not so much. My adult daughter set off the metal detector. They wanded her, and the wand went off. They then waved her through. So what exactly did that do?
What I hate about it is that it is an inconvenience that does nothing to actually make the park more secure. To illustrate this - before the park opened all adults went through. Strollers with 50 lb bags of unidentified stuff underneath them - not even attempted to be scanned. The same goes for the bag looking. My tiny crossbody bag - looked into. A reusable plastic popcorn bucket that can't be seen into and has a closed lid and is big enough to hold at least one handgun - not given a glance. You either have to diligently check everything or it's about the same as checking nothing.
Also - some of the worse cast members that I have met have been working at the metal detectors.