Sherlock picks up on the non-verbal cue. He returns the box. She didn't give him a case, per se, but he's not going to turn down her offer. In his mind, he will view it as a type of payment.

"Thank you, Marie. Such a place could prove useful as I continue to adjust to being here."

He offers her a faint smile.

"Good luck with your deliveries. And if you need me, I'm sure these PINPoints are useful for that."