My guess is that the missing angel died and others ate him and you may find remains. For some reason, remains, because they have no or little color are not as noticeable to me. Sorry about that. If the angels are not that old then it can be a bit shocking when their behavior starts to change as they. I have a pair that were just cute as heck when they were little. Now they spawn every 2-3 weeks and right around there they do some battling. I was worried enough last week I shut the lights out to calm them down. Then they get over it and they are buddies. I had another two in another tank and they were nasty. Two of them started hunting neons, killed one and an oto, or at least harassed them to death. And these angels were only about an inch across the body. I only have one now, gave the other to the lfs, and that one is pretty nasty to the apistogrammas in the tank. What I'm trying to say is they do get nasty for many reasons I suppose and there are plenty of stories about them killing one of their own.
Sorry for the news but in part that is a cichlid for you.