BEC means "battery eliminator circuit". The receiver takes it's power source (5-6V) from the ESC instead of from a separate receiver battery pack. The only way to be absolutely sure the motor won't start up on its own is to disconnect the batteries. They can randomly start from time to time even when the receiver is turned off. Keep your fingers away from the prop if the battery is connected. I wouldn't conclude that your ESC is defective right away. If it continues to be a problem then maybe it is.

Greg