3 ideas
High-school amateur astronomer Aki Shiraishi observes an anomalous structure rising from Mercury. It becomes a vast ring encircling the sun, and its shadow cuts sunlight to Earth, causing global cooling, crop failure and social crisis.
When an unknown agent will not or cannot communicate, its goals can be reverse-engineered from the physics of what it builds. A structure's orbit, mass, materials and energy budget reveal what it is optimized for.
The gravest threat from a vastly more capable agent may come from it not noticing you, not from malice. Harm arrives as a side effect of pursuing unrelated goals, as with the ring's shadow freezing Earth. Diplomacy and deterrence assume an adversary who registers you, so both fail against an indifferent actor.