What indicates that a shield is not properly bonded?

An erratic signal on a device is a strong indicator that a shield is not properly bonded. Bonding ensures that the shield effectively protects the devices from electromagnetic interference (EMI) and maintains a consistent ground reference. When the shield is not properly bonded, it can lead to fluctuations in the signal as external noise or interference gets coupled into the system, causing the signal to vary unpredictably. This erratic behavior reflects the failure of the shielding to provide the necessary protection, resulting in inconsistent performance of the electronic device.

In contrast, a constant signal could suggest a different issue, while no signal or signal loss during operation may point to a more severe disconnection or failure in the system rather than just a bonding issue with the shield. Thus, the presence of erratic signals directly correlates with improper bonding of the shield, making it the correct choice in this context.