The chapter which contains this episode also contains a scene where Minerva is later interrogated by a member of Trujillo's regime who clearly has the letters in front of him. This comes after the dance, when Minerva and her family have left the party before Trujillo and thus risked his displeasure. As a result, Minerva's father is taken in for questioning. When Minerva and her mother go to the capital to be close to him, Minerva herself is arrested. Note what she sees when she goes into her interrogation room:
He opens the file in front of him. From where I sit, I can see the pink registration slip from the hotel. Then a number of sheets of paper I recognise as Lio's letters from my purse.
It is clear therefore that the letters Minerva left in her purse were discovered by somebody, though it is doubtful whether it was by Trujillo himself, yet whoever found them has given them to Trujillo. However, clearly this is why Minerva is being interrogated at this point in the novel. The discovery of the letters and her connection with a known revolutionary places her in a very precarious position, and this, combined with her father's arrest and Trujillo's obvious sexual interest in her serves to leave her in a very dangerous position.