Jamieson, Fausset and Brown on Leviticus 5:5

5. it shall be, when he shall be guilty ... that he shall confess that he hath sinned in that thing--make a voluntary acknowledgment of his sin from the impulse of his own conscience, and before it come to the knowledge of the world. A previous discovery might have subjected him to some degree of punishment from which his spontaneous confession released him, but still he was considered guilty of trespass, to expiate which he was obliged by the ceremonial law to go through certain observances.