Sforno on Numbers 5:13/ספורנו על במדבר ה׳:י״ג

1ושכב איש אותה, and another man has lain with her It is characteristic of the evil urge to cause the sinner to proceed from a relatively mild sin to ever more serious sins.

2ונעלם מעיני אישה, even though there had been indications of mistrust, jealousy, etc., her latest indiscretion remained hidden from the eyes of her husband. It is as if his eyes had been too weak to see what was going on. Had her husband been aware of her infidelity, the waters would not even have the power to bring this to light. Our sages have made this quite clear (Sifrey 7).

3ונסתרה, she had subsequently hidden, and this fact had become known to her husband.