Like a thief in the night simile
Used to indicate the way someone does or something happens is secretive and quick
You need to quit smoking or illness will come like a thief in the night.
She didn't want to wake the baby up, so she moved like a thief in the night.
A thing is known about by only a few people and kept hidden from others because it makes somebody feel ashamed.
To be secretly working together
A person who contributes important work to the organization but is kept secret.
To secretly punish or rebuke someone for doing something.
Do something silently, secretly.