Many modern brake pads have a simple circuit of a wire connected to the computer. The wires are designed to break when the pad is worn out so the car computer detect a voltage return of zero in that electric circuit and will trigger a warning immediately.
Hypothese 1. If the maker forgot to install the brake pad. Then we have to assume that neither the second employee nor the car computer noticed that something was wrong, and this is very strange because the computer at least -must- know it.
Hypothese 2. The pad was installed but was loose or fell later. The seller or one of the seller employees had some fun driving the car before, hit a bump or something and the pad was dislodged and fell. They didn't noticed it or tried to hide the problem.
Does not explain why the owner didn't noticed a warning in the dashboard from the minute 0.
Hypothese 3. The owner hit a bump and lost the brake pad in the first days of driving the car. Maybe without being aware of that. So nobody see the warning light because there was not a warning light to be seen. Not until later. (we could also suspect that the structure that keeps the brake pad in place maybe is fragile in this car).
There are other possibilities, but they are much less probable