Ok, so this is not actually a pet shop AU but it is a animal rescue center AU, which I think is much more in keeping with the characters.
"Oswin, have you fed the new arrivals yet?" asked Cadfael as he loaded the washing machine in the laundry room, keen ears able to identify which member of staff was approaching by their footsteps.
"No, not yet," the passing teen paused in the doorway and replied, "I got distracted with the kittens. You know that ginger one keeps hiding in the blankets and I keep thinking it has escaped so I went in and-"
"Yes, Oswin," he interrupted. "I get the picture. Now go feed everyone in quarantine before they start howling in hunger." The teen hurried off and Cadfael sighed.
"Business as usual then, Cadfael?" came a voice from behind. Cadfael turned in surprise.
"Hugh!" he exclaimed, delighted. "I wasn't expecting you here today, you're not scheduled on the volunteer roster until the weekend." Hugh ducked out from under the open window that he had stuck his head through and came round through the staff only entrance of the utility room next door.
"Well, I was passing and I thought I would pop in and see how our newest guests are doing."
"And…" Hugh looked at the worming medicine leaflet stuck to the notice board but said nothing. Cadfael fixed him with a stare. "The pups are the same as when you and those detective constables dropped them off yesterday. I know that's not why you are here." Hugh sighed.
"And I need your help with a case. If you've got a moment."
"I've always got time for an old friend, you know that. Oswin can handle the center for a little while. Let's get to my office, we won't be interrupted in there."
"Thanks, Boss." Cadfael scowled at him.
"I stopped being your boss when I retired from the police." Hugh gave him a cheeky smile.
"I know!"