My memories of Prague are from about 10 years ago when I (for a few months) needed to fly back and forth there for a project.
If the metal detector beeped they'd tell you to try going through again but to go faster. Wonder if they still take that approach?
I'd hope the speed you went through the metal detector wouldn't affect the detection capability so I wonder if they just made you do that so they wouldn't need to do a pat down if it was a random beep.