In a recently discovered rare species of Seyfert-2 galaxies we observe ultra-luminous, galaxy-wide narrow-line regions.  Their [OIII] emission is so high that they outshine their host galaxies. Using VLT/XSHOOTER we observed the narrow-line region in one of these galaxies in detail, and find that they are unlike any other such region known. More galaxies with similar properties were identified in SDSS and confirmed with Gemini South. Based on this larger sample and the galaxies' mid-IR luminosities, we show that the current nuclear activity is insufficient to explain the luminosities of these narrow-line regions. Our best explanation is that we are observing an ionization echo from an earlier, much more active quasar state, which has recently subsided. This opens a new window to study the shut-down times of quasars on time-scales much longer than a human life-time.