I work on my cars as a hobby (not a mechanic). All the brakepad sensors I’ve seen were only on one pad (usually the inner one, as that wears off first). As for your 2 and 3, there is simply not a chance that a bump will make a pad fall off, unless the whole brake system has major damage.
The explanation is usually the simplest, they forgot to install it.
The explanation is usually the simplest, they forgot to install it.