Mine hide all the time. Sometimes I don't see them for a week. I check the tank late at night but they hide pretty good. Every once in a while I get a glimps, so I know they are still there. Just a new place for them, I got them a couple three weeks ago and I seen them just 3 times since I put them in. I saw them tonight when feeding Mysis to the rest of the tank. They came out for a brief second to snatch some food and then disappeared again.
125 Gal-35 Gal Wet/Dry-Tetra-Tec PF500 HOB
(3) AuquaClear 301 Powerheads-600 GPH
Automatic Tempature Controller
5-6 Inch Sand Bed-200+ lbs of LR