One issue with that is based on current physics, a deep space ship that isn't some millenia generation ship but accelerates to a decent percentage of c will expel a fuckton of energy in some type of fusion/fission/antimatter reaction, which tends toward being very, very visible. The first sign of a torchship-capable civilization wouldn't be random radio emissions, but would instead be some star system suddenly gaining another star that ignores orbital physics and instead ends up leaving said system at some absurd speed, probably with a bowshock measured in AUs from all the interstellar dust going nuclear as it collides with said ship.