I have an ESD mat and wrist strap. The wrist strap plugs into a little box that beeps when either the mat or the wrist strap do not have a good connection to ground.
I also have a standard Phihong 5V wall wart. I have noticed that when the wall-wart is plugged into a device, and the device is sitting on the mat, the box indicates a fault with the mat. If I touch an exposed ground pad on the device, I get another fault indicator for the operator, as if my wrist strap is improperly grounded.
If I attach a serial cable to the device, this problem vanishes. I assume this is because the serial cable somehow provides a proper ground to the device.
This happens even if the device is off. However, the wall-wart is still active; if I turn the wall wart off by flipping the switch on the surge protector, the problem vanishes.
The problem also appears sometimes when I merely approach my mat with the device connected, without even necessarily touching it.
Is there an explanation for this odd behavior?