Building Wiring Fault - is LED on or faintly dim?

[BillP]

I'm using an older APC Back-UPS NS 1080 to backup PC and monitors.  works fine, wiring fault light is not on.

I've also plugged in a new APC Back-UPS 1100 pro to the same outlet to initially charge the batteries.  The wiring fault LED is faintly glowing when the unit is on or when powered off, but still plugged in.  Then the LED goes completely off when unit is unplugged.

Is the faintly lit LED on the 1100 just in maintenance mode?  When I power down the UPS 1100, the LED goes completely off.

When I swap the two UPS to plug into the other receptacle of the same outlet, everything is the same.

If there is a wiring fault, will the LED shine bright red? or is the dimly lit LED on the 1100 telling me something?