Our existence in the Universe resulted from a rare combination of circumstances. The same must be true for any advanced extraterrestrial civilisation. If there exist any in the Milky Way, they are likely scattered over large distances in space and time, however, they must be aware of the unique property of the Galactic centre: it hosts the closest massive black hole to anyone in the Galaxy. A sufficiently advanced civilisation may have placed material in orbit around this black hole to study it, extract energy from it, and/or for communication purposes. In either case, its orbital motion will necessarily be a source of gravitational waves. Here we show that a Jupiter-mass “Messenger” on the innermost stable circular orbit around the black hole can be sustained for a few billion years by the energy output of a single star and emits an unambiguously artificial (continuous) gravitational wave signal that will be observable with LISA-type detectors.