Precisely. It’s just that they kill thousands of people with each attack.It's probably the Unknown Probes/Unknown Artefacts. These things are probably still transmitting albeit on a frequency we can't detect. I would say the Thargoids are simply responding to these transmissions. They follow them to the source, calculate that the origin point is inside one of these alien (to them) structures and figure, "Let's show these people what happens to people who steal our technology. Don't kill them all, just scare them snotless."