Gas Thieft - 2000 ZTS

My Aunt has a Ford Focus 2000 ZTS 4 door.
Someone is staling her gas at night. A couplle times she has come out i the mronng and found the gas door open and gas cap hanging.
Does anyone know how they are doing this (I thought they started putting some kind of wire in the gas filler tube tp [revent a hose from being inserted years ago) ? Also, besides a locking cap what can be done to stop it?
Any and all help will be appreciated.

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