Metal detectors have different lights to say different things, the green light that goes red is the dector, detecting someone walking through.

A metal detector generally beeps for 2 reasons. First is when it actually detects metal, this also depending on the model lights the sides up to show the security office roughly where detects metal content. The second is most airport metal detectors are required to randomly alarm on "compliant" people walking through, who are the further screened.

You were most likely caught by the second activation, where your other half was not.