Bob Harris, an aging American movie star, arrives in Tokyo to film an advertisement for Suntory whisky. Charlotte, a young college graduate, is left in her hotel room by her husband, John, a celebrity photographer on assignment in Tokyo. Charlotte is unsure of her future with John, feeling detached from his lifestyle and disillusioned about their relationship. Bob's own 25-year marriage is strained as he goes through a midlife crisis.
Each day Bob and Charlotte encounter each other in the hotel, and finally meet at the hotel bar one night when neither can sleep. Eventually Charlotte invites Bob to meet with some local friends of hers. The two bond through a fun night in Tokyo, welcomed without prejudice by Charlotte's friends and experiencing Japanese nightlife and culture. In the days that follow, Bob and Charlotte's platonic relationship develops as they spend more time together. One night, each unable to sleep, the two share an intimate conversation about Charlotte's personal troubles and Bob's married life.
On the penultimate night of his stay, Bob sleeps with the hotel bar's female jazz singer. The next morning Charlotte arrives at his room to invite him for lunch and overhears the woman in his room, leading to an argument over lunch. Later that night, during a fire alarm at the hotel, Bob and Charlotte reconcile and express how they will miss each other as they make a final visit to the hotel bar.
The following morning, Bob is set to return to the United States. He tells Charlotte goodbye at the hotel lobby and sadly watches her walk back to the elevator. In a taxi to the airport, Bob sees Charlotte on a crowded street and gets out and goes to her. He embraces the tearful Charlotte and whispers something in her ear. The two share a kiss, say goodbye and Bob departs.

Question: What leads Charlotte to get into an argument with Bob over lunch?
Answer: Charlotte finds out Bob slept with the bar's jazz singer
Alternative answer: She knows he slept with the jazz singer.