The "stealthy" civilization has ceased to be stealthy, in this case. Launching an attack of any kind defeats the purpose, if anyone but the fully-wiped-out target can see. Your example suggests that a stealthy, hostile civilization isn't possible.

What would an observer see? It’s plausible that they would see an attack from an uninhabited solar system but not be able to find the home location of the attacker.

Ooh, that's the fun part. What (augmented) sense does the observer rely on? On what timescale? How was the attack coordinated? The attack surface of the operation - and the ability to trace it back to its origin - might be wider than you can plan for. "Plausible" is a shaky ground to be on with your civilization in the balance, if you suspect there's even a chance you can be discovered and have chosen to remain as hidden as possible heretofore. Dark Forest doctrine and hostility are incompatible, I think.