ARA obtained motors may come from a design in which the case of the motor is tied electrically to one of the wires or may actually be the only return connection for the electrical current. A motor with only one wire is a dead give away. Please be sure to measure your motors and insulate the case as needed. This may mean mounting them using polycarbonate, insulated hardware or insulated standoffs. If the case of the motor or attached transmission is plastic, current may flow through an attached metal gear so check that as well.
Thanks from your inspection team.