Maruti Suzuki India — swift car starts rusting in 2.5 years

I have noticed the paint is bubbling up on the driver side door because of rust. I have visited the dealer and he suggested to pay money and replace the door. The car is under extender warranty and i told him like, no car used in bangalore should start rusting in this time period. Because i did not get help from the dealer, i escalated this issue to maruti (Interaction id [protected]). Maruti responded very quickly and they inspected the car again. I got a call from maruti on 25/01/2019 saying there is no manufacturing issue, and the rust could be because of external factors. He was suspecting the environment, water used to wash the car etc.. And the complaint is closed now.

I also would like to mention the below points.
1) if the water is the culprit, there are multiple cars in my apartment washed with the same water for last 8-9 years and those cars doesn't have any issue.
2) why the rusting issue is limited only to one door? It does not have any rust marks in any other part.
3) if it is the water issue, the top portion should start rusting first. Why this started for the door?