When Moses came down Sinai and saw the calf, he did not only smash it. He burned it, ground it finer than any mortar should grind gold, and then he did something stranger. He scattered that dust across the surface of the stream and made the children of Israel drink.
Targum Pseudo-Jonathan, the Aramaic paraphrase of the Torah attributed in tradition to Yonatan ben Uzziel, explains what Moses was doing. The powdered calf was a trial. Whoever had contributed his gold trinket to the molten idol, the gold returned to him, surfacing as a mark upon his nostrils (Exodus 32:20). The stream became a courtroom. The guilty could not hide what they had given.
The image is severe, but notice its mercy. Moses did not ask the camp to confess. He did not call out names. The sign came from within. The sin that had been poured into a mold, the trinket torn from an earlobe, the offering made in the haze of the golden light, all of it returned to its giver as a visible witness.
When the Levites later went through the camp with swords (Exodus 32:28), the Targum says it was these marked men who fell. Justice had been written on their faces first.
Takeaway: A sin given freely does not stay hidden. In Jewish thought, teshuvah begins when we see the mark we carry and choose, at last, to wash it off.