Lilian Jackson Braun: The Cat Who Wasn't There (The Cat Who... Mysteries, Book 14) [1993]

Persuaded by his beloved companion to join her in a group tour of Scotland, Jim Qwilleran expects to revel in his Scottish heritage while keeping Polly Duncan safe from the Pickax Prowler. Instead, his trip is cut short when a thief swipes a suitcase, the bus driver disappears, and a fellow tourist is found dead - all in the same day. The distraught tour members return to Pickax, only to find the town in a tizz. And when village scuttlebut focuses on the bizarre behaviour of one of the tour's surviving members, Qwill's sensitive moustache tells him that more trouble is on the way.