quiet as a mouse

(as) quiet /silent / still as a mouse (very quiet) — тихий как мышь; затаившись как мышь; ≈ тише воды, ниже травы
Just as a mouse may be quiet because it is timid and afraid of being discovered, so a person can be quiet as a mouse to avoid being caught or discovered.

Example: Keep quiet as a mouse and don’t tell our secret to anyone.

Example : The boy was so shy and timid, he always remained quiet as a mouse, even when the rest of the kids were noisily joking, and kidding around.

Example : Just hide in the closet and keep quiet as a mouse! Or I’ll get in trouble for having unauthorized guests after curfew.

Related vocabulary:
[meek and mild]
[keep a low profile]
[make oneself scarce]
[lay low for a while]