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.