There are other techno-signatures to look for: Dyson swarms (marked by irregular dimming). Infinitely more powerful transmitters - i.e. irregular pulsars. Oumuamua type fly-bys. "Unnatural" gasses in atmospheres.
All purely theoretical, of course. As far as we know, we might not even notice life on Alpha Centauri planets 4ly away.
I think a big Orion drive powered spaceship flight profile would be widely noticeable. Like, nature rarely produces hundreds of sequential nuclear detonations in deep space in quick succession, possibly perfectly matching high energy transfer orbits.
Just not sure what would the other party thing about builders of such craft.
On the other hand, its clear what would they think about anyone running a Nuclear Salt Water Rocket - eq. riding continuous fission detonation for minutes or hours! They would definitely think they are batshit insane! :D
All purely theoretical, of course. As far as we know, we might not even notice life on Alpha Centauri planets 4ly away.