How can I determine the presence of live AC wires behind a wall with this stud sensor?

Holding it as instructed, slowly move the sensor across the wall while pressing and holding. If the AC LED illuminates, you are near 6-18" from active AC wire; AC LEDs along with center red LEDs indicate AC voltage at stud's center when detected (Pages 5-7).

From the official Stanley FatMax PP1DCS Portable Power Station Jump Starter manual, page 7

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