After relocating to Zurich last spring to run Credit Suisse Group AG as chairman, António Horta-Osório flew to London and Lisbon for work meetings and business events, then spent time with his family at homes there.

On multiple occasions, a private plane paid for by Credit Suisse dropped him off on a Thursday and returned empty to Switzerland, according to people familiar with the travel. By Monday, a plane picked Mr. Horta-Osório up for work in Zurich.