I think I prefer it when they disappear rather than getting stuck in a rock so I have to take them out, it sorts of makes it much sadder...I have just lost my male Lyretail Anthia in the same way. He started off just resting on the sand, in the exact same spot, for short periods. Those periods gradually became longer and longer. His eating became more and more sporadic. Then one night, he just disappeared. This happened over a period of about three or four weeks. During this timeframe he also seemed to have lost a bit of his colour, and when resting on the sand he appeared to be having problems breathing. His little mouth was open and his Gills seemed to be working overtime. Was it an internal parasite/infection? I don't know.