The Adultery is Hidden from Her Husband and There Is No Witness (Numbers 5:13).
“ And it be hid from the eyes of her husband, and be kept close, and she be defiled, and there be no witness against her, and she be not taken in the act,”
The emphasis here is on the fact that this grave sin has been kept hidden, although Yahweh will know about it. Her husband does not know, she deliberately keeps it hidden, there is no witness against her and she has not been caught in the act. But she has been defiled. And because she is defiled the camp is defiled. All may suffer because of her sin.
All of us need to recognise that when we carry around secret sin we defile wherever we go. Many a church's witness is marred by the secret sin of its members.