From what I've seen over the last few years, the metal detectors in many airports also serve the purpose of randomly selecting for body scans and/or explosives

From what I've seen over the last few years, the metal detectors in many airports also serve the purpose of randomly selecting for body scans and/or explosives checks. I imagine it's an attempt to reduce discrimination and bias by humans. It's likely that had it detected metal it would have made a different sound / colour. What you heard was probably the RNG landing on "check them for explosives".