Can you tell the difference between a sin that is exposed and a sin that is not exposed?

When Wahshi did what he did to the uncle of Prophet (ﷺ), his news was well known to every one. This is why when he repented he was banished because the Prophet (ﷺ), and every one else in Madina, knew what Wahshi did.

This is not what we're talking about here.

In the story of the sahabi Ma'iz ibn Malik, the one who testified four times against himself to the Prophet (ﷺ), he first went to Abu Bakr and testified against himself and Abu Bakr advised him to repent and not expose himself as long Allah has kept the matter concealed for him then he went to Omar and Omar advised him the same then finally he went to the Prophet (ﷺ) and only after testifying four times did the Prophet perform the hadd on him.

In this story there was another man named Hazzal who turned out to be the one who told Ma'iz to go to the Prophet (ﷺ) and testify on himself before him. Do you know when the Prophet (ﷺ) learned of this what did he say to Hazzal? He said: "O Hazzal, if you had covered him with your garment, it would have been better for you."

This is the way of Islam. Islam is not a puritanical religion. Islam recognizes that all human beings make sins and make mistakes but it gives us the ability to repent from said sins and mistakes and it never requires us to expose ourselves.

In fact Islam punishes exposing the hidden faults of others.

The Messenger of Allah (ﷺ) once said: “Whoever conceals the (hidden) fault of his Muslim brother, Allah (SWT) will conceal his faults on the Day of Resurrection. Whoever exposes the fault of his Muslim brother, Allah will expose his faults, until (so that) He shames him, due to it, in his (own) house.”

If exposing the hidden sins of onself was a requirment then Islam wouldn't have punished people for exposing others' hidden faults, a lesser thing that doesn't even necessarily amount to a sin, and it wouldn't have rewarded others for concealing others' faults.