I leave the red wire in the plug since it doesn't have any bad effect on the BEC. The ESC will light up when you plug in the receiver pack and the LEDs will ramp up and down but the motor won't work unless the main battery is connected.
Well, when I connect the receiver battery to the receiver, the servo and esc work and the lights ramp up and down in response to transmitter inputs, and also the esc will activate the motor from the receiver battery power even if the main battery is not connected. Yes the the motor does indeed run from the power of the receiver battery through the esc receiver plug, not fast but it does run. This is a bad effect in a sense that the motor does draw a bit of current from the receiver battery pack.
When I remove the red wire from the esc receiver plug, and power the receiver with a separate receiver pack and the esc with a separate 1s main battery, the lights on the esc will ramp up and down in response to transmitter inputs but the motor does not run. That weird!
I guess that's just how this esc is designed. All aircraft esc I have used in the past will work properly even when I remove the red wire from their receiver plug. A proper practice when you want to run a separate receiver battery pack for extra reliability purposes, or unload the extra work from the esc.
It look like this esc is screwed if its bec ever decided to go out.