For all we know, there could be a gigantic tungsten rod slowly and purposefully accelerating toward us that would destroy all life on earth in ten thousand years, and we'd have no way of knowing.
Imagine you are in impactor frame of reference -- any particle of dust it meets will cause tremendous damage to it regardless of what the impactor is built of (assuming our existing technology).
It is in my opinion very likely that over long distances the impactor will just get converted to a shower of high energy particles over very large area.
Although I guess they could also just paint their weapon black.
A different matter how they hide the engine's infrared signature. Perhaps by just not running it when approaching, except for the final course corrections.