The janitor was mopping the main hall of the submarine, and no one was really paying any attention to him.
He went unnoticed as he mopped.
He went unnoticed as he crept over to the wall, and placed his final plastic explosive on the bottom of the wall.
He went unnoticed as he kept mopping the halls, and walking along, until he reached the captains quarters.
He went unnoticed as he pulled a stolen key out of his pocket, unlocked the door, and quietly walked in.