Where there is enough evidence to cause distraction or scandal to the flock and to directly confront such individuals.
If you know, then others in the parish know, which is then public information. I am not sure why you believe that this then becomes a witch hunt.
You stop short of presenting the whole gospel message of salvation calling for repentance in order to receive the saving redemption of Christ’s blood:
“For there is no distinction; since all have sinned and fall short of the glory of God, they are justified by his grace as a gift, through the redemption which is in Christ Jesus, whom God put forward as an expiation by his blood, to be received by faith.” Romans 3: 22-25
“I have not come to call the righteous, but sinners to repentance." Luke 5: 32
This is an inadequate response, IMHO. You know that a present darkness occupies the choir loft, yet you allow this to go on unopposed. Take a lesson from St. Paul who made no bones about exposing darkness:
“And walk in love, as Christ loved us and gave himself up for us, a fragrant offering and sacrifice to God. But fornication and **all impurity **or covetousness must not even be named among you, as is fitting among saints. Let there be no filthiness, nor silly talk, nor levity, which are not fitting; but instead let there be thanksgiving. Be sure of this, that no fornicator or impure man, or one who is covetous (that is, an idolater), has any inheritance in the kingdom of Christ and of God. Let no one deceive you with empty words, for it is because of these things that the wrath of God comes upon the sons of disobedience. Therefore do not associate with them, for once you were darkness, but now you are light in the Lord; walk as children of light (for the fruit of light is found in all that is good and right and true), and try to learn what is pleasing to the Lord. Take no part in the unfruitful works of darkness, but instead expose them. For it is a shame even to speak of the things that they do in secret; but when anything is exposed by the light it becomes visible, for anything that becomes visible is light.” Ephesians 5: 2-13