Prompt from Domina Temporis: Explain something in Canon that you've always wondered about

My story around Kate and Isa Whitney from TWIS made me remember something I've always wondered about - what did Mary think when John didn't return from finding Isa?

"John said he'll send Isa home, and so he will," I said in a reassuring voice, my hands holding Kate's as she shuddered with worry. She needed some steadiness, and I knew that John's word could be counted on. If Isa was at that den of sin, John would find him. Even if he wasn't, if I knew my husband, he would continue searching for Isa until morning, just to ease Kate's mind.

Kate nodded rapidly. "Of course, Mary, of course. I just worry so. He's never done this before."

I kept silent. Isa may not have stayed out so long before, but it was far from the first time the addiction had taken hold of him.

It couldn't have been more than an hour later when there was a sharp knocking on the front door.

Kate startled, awakened from the light doze she had slipped into. "Is it John? Mary, is it John? Has he found Isa?"

"I'll just go check," I soothed her, and went to open the door.

Outside was a cabbie, his cap still waiting for him in the lane. "You Mrs Watson?" He asked.

"Yes, I am," I replied, curious. Had something happened to John, or Isa?

"Got a message for you from your husband. He got me to send his friend home, then said to give you this note." He thrust a scrap of paper towards me, and I took it.

It read, 'Am out with Sherlock. Will be back when I can. Love, John.'

I thanked the cabbie with a smile, and hurried back inside. "Kate, Isa's home! John just sent a cabbie by to say he'd dropped him off."

"Oh, thank you!" Kate wrapped me in a hug, then flew out the door, calling behind her, "Thank John for me when he returns!"

"I will." Whenever that would be. I had known when I married John that Sherlock was an important part of his life, and never begrudged him that. Without Sherlock Holmes, we never would have met at all. Yet sometimes, on nights like tonight, it was hard not to feel alone.