For several days in September, a private investigator hired by Credit Suisse trailed the bank’s former wealth management head, Iqbal Khan, through Zurich, snapping photos along the way. The mandate: Find out whether he was trying to recruit colleagues to his new employer, UBS.

On September 17, Khan spotted the investigator, people briefed on the events said, triggering a scandal that followed months of tensions between the banker and his former mentor, Credit Suisse chief executive Tidjane Thiam.