Haha, I had a fish one time that would disapear during the day and when the aquarium light was off it reappeared. I couldn't ever catch it in the process of hiding it was always there or not. It was in a small aquarium too so that made it even more mysterious. I named him the ghost fish cause of that.
otos are great little hiders. I had 4 in a tank and just assumed 2 of them died because thats all I ever saw at once. sure enough some spinach brought them all out. my bamboo shrimp will hide for long periods on occasion as well.