How to locate hidden joints when pressure drops during a plumbing rough-in pressure test?

This week, during a plumbing rough-in for a small commercial project, the gauge pressure slowly dropped from 100 psi to 88 psi. It wasn't a fast leak, but we definitely couldn't close the walls with an unresolved issue. We couldn't hear any obvious leaks on-site, so everyone initially suspected a faulty pressure gauge. I first swapped it with a calibrated gauge to confirm it wasn't an instrument…