There are sprinklers where this happens and the sprinklers look exactly the same. There’s a pressure switch on the sprinkler line that activates a deluge pump. This pump has enough pressure and flow capacity to break open the glass ampules of the remaining sprinklers in the circuit.
Yes. A combination of rust, thread cutting oil, and water that has been in the pipes often since the system was filled. It smells, it will stain anything it touches, and it's a smell that's difficult to remove.
Not true everywhere. Many buildings, especially industrial, require a flush of the fire suppression system annually or biannually to test that everything is still functional.