(as) quiet as a mouse
Silent, meek, and gentle. I just can't believe that Kate actually yelled at Mike—she's usually as quiet as a mouse! We remained as quiet as mice to avoid being detected by the guards.
*quiet as a (church) mouse and *quiet as the grave
very quiet. (*Also: as ~.) You'd better be as quiet as a mouse while Grandma takes her nap so you won't wake her up. This town is quiet as the grave now that the factories have closed.