might be something/vapour in the air that gets settled into the water already. has anyone been spraying deodorant or insecticide? check with your family members as well coz u mentioned that you're having consistent good parameters and this happens. although skimmer can skim out everything.... whatever that got into your tank, got to your fishes and corals first before being skimmed out.
cannot be from within the tank or any of the sand or rocks itself coz u also mentioned u had a dish of water in the room once before and it gave out similar smell after being left there.
IMO, whatever it is, its not coming from the tank or its content.
try leaving windows open for ventilation and get your tank covered with a proper hood or something. Good luck!