Z3 Fail NJ Inspection for leaking gas cap

Here in NJ, the motor vehicle inspectors only check emissions either at the tail pipe on older cars or using the on board diagnostics via the OBD port and for any leaking of the evaporative gas system. My 98 Z3 2.8L passed the OBD tests, but failed for a leaking gas cap. My question is if the gas cap was leaking, shouldn't it have thrown an error, i.e. "small evaporative leak" Has anyone else seen this?

Thanks,
Al