did your mechanic perform a voltage drop test to see what is draining the battery ?
I would ask the repair shop to perform this test to find out why .
Voltage drop test ,for battery drain checking
2- settings on voltmeter will be on milli amps.
3- go across all the fuses in the car one by one and see which one is reading higher than 0.3 0.5 milli amps with the key off
4- the fuse that has a reading higher than 0.5 milli amps will be the circuit we need to repair to fix the battery drainage problem you having
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ASE Certified Technician