Im sure I'm not the only one to have one. Corals are all closed up, couple fish are missing, all water tests are perfect, remaining fish look fine, ah the joys!
^ Happens to me on a regular basis, today it was the sensor to the ATO, yesterday the air hose to the skimmer, two days again the impeller to the return pump. This is reason to have back ups.
Hope your day gets better and you find your missing fish a live.