FREMONT (CBS SF) – Thieves are cleaning out unsuspecting ATM customer’s accounts through a method known as ‘shoulder surfing.’

Last month, a Fremont woman notified police after her account was debited during ATM transactions she didn’t make.

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The 53-year-old said she made a single transaction at an ATM machine, but the bank called later and said two withdrawals of $200 were taken from her account after she walked away. Police are certain she was a victim of ‘ATM shoulder surfing.’

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Authorities warn this kind of theft occurs when customers leave the ATM machine before the session is completely closed. The screen asks if they ‘want another transaction,’ a waiting thief steps up, takes over the machine, and withdraws more money from an account. Often a PIN number is needed to complete the transaction, but if the thief has been watching the ATM customer closely, they often manage to observe the PIN number.

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Authorities investigated two ‘shoulder surfing’ thefts at the same ATM earlier this year, and similar incidents an ATM in Milpitas. A woman was arrested after someone observed her standing near the ATMs, and watching ATM users from a parked car.