In front of people, she may throw the compliment away.
An eye roll. A sharp little “please.” Maybe a joke tossed across the table so nobody can say she enjoyed it. The performance is quick, almost bored. But later, the sentence is still sitting there intact.
She remembers whether he said “the dress” or “that dress.” She remembers if he looked down after saying it. She remembers the doorway, the glass in her hand, the friend who suddenly became very interested in her straw.
Trash does not usually get filed that carefully.