Oh, and I'll second the filter suggestion. Red cherries, for one reason or another, love to get sucked up filter tubes. They must think its some sort of carnival ride.
Is there any way they could have climbed out of the tank? Like, is there a rough background (such as a moss wall) that they could use to get out? I've never heard of cherries doing this, but I know of some other inverts that, for one reason or another, jump ship.
Also...its possible you're just missing them. If they shed, they may get a little reclusive and hide under rocks/whatever. My amano shrimp disappear for weeks at a time...and this is in a small (20g) tank.