See Delphi and Die by Lindsey Davis

The 17th book in the bestselling Falco detective series.
Humour, surprises and domestic irony await Roman sleuth Falco as he attempts to solve the disappearance of two newlyweds on their honeymoon trip to Ancient Greece.
With safe seas, good roads, a common currency, and lots of interesting conquered territory, the Romans naturally invented tourism. They believed in a life of leisure — fine for those who could afford it — and some would set off on their travels for maybe five years at a time. As these ancient culture vultures descended on the sights, the tour guide was born to cater to their needs.
“Marcus, you must help me!” Stunned by the dramatic appeal from his otherwise cool mother-in-law, Marcus Didius Falso cannot resist. His brother-in-law, Aulus, has been diverted from his route to Athens University by a man whose newly married daughter and her husband have disappeared while visiting the Olympic Games as part of their wedding trip. Suspecting a classic cover-up, Aulus enrolls Falco’s help in solving the case. And of course his mother-in-law hopes to hurry her son along to university by passing the case over to Falco.
Joining the rest of the married couple’s tour group on the remains of their Grand Tour, Falco and Helena seize the opportunity to interview the owner/manager of “Seven Sights Travel,” as well as the other guests. Seemingly not getting very far, they can at least make the most of the splendid sights; but finally, on reaching Delphi, Falco and Helena unravel the mystery of the bride and groom.