That’s to test the incoming main, the actual grid on the floor doesn’t get flushed. There’s a lot of dead end pipes that can’t be flushed.

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.

This happens with fire sprinklers a lot, one sprinkler goes off, and triggers the rest of the floor, or sometimes even building.

That’s not how it works. Each sprinkler has it’s own trigger mechanism, the glass bulb, and cannot trigger another sprinkler.

There are systems where this happens, but the sprinkler heads look very different, and you won’t find them in an office building.