Trust Sherlock to get involved in a case when he and John were on a long-awaited holiday. Trust him to get seriously invested in it and crawl around the crime scene to solve the murder of Mr Jonathan Cartwright (retired lawyer, passionate gardener, universally liked), unsettling the local police. And trust Sherlock to solve the case by finding some obscure clue – on the paw of the victim's dog, which he'd found huddled under the sofa.
So, after Sherlock had informed the police about the crime's circumstances (illegitimate son with a grudge, boring, really John, why did I bother), they stood outside the victim's house, looking at the dog at their feet. They had been told that it would go to a shelter, since there was no one to take care of it. John had always liked dogs and this friendly tail-wagging fellow was no exception. So he'd said, not to worry, they would take him.
He hadn't been thinking. But Sherlock had remained conspicuously silent, not even raising an eyebrow. Perhaps he found the dog interesting because it had been involved in a murder. One never knew with Sherlock.
The dog returned their scrutiny, tail still wagging, tongue lolling out, obviously quite happy.
Well. Trust them to finish a case during their holiday with taking home the "evidence": a bulldog.