When Holmes examined the arsonist's body, he set his gun on he table out of habit – the habit of testing Watson's reliability. A test whose outcome was never in doubt.
Watson's quiet but unusually firm, "You, Holmes. Not us." froze him at the door for a long moment before he could respond,
"A figure of speech, Watson."
And duck out the door. Halfway down the stairs he realized he'd left his gun. Any other day, he'd be confident Watson would catch him in a block, completely reliable. After the cell yard … Holmes would never admit to a moment of doubt as he walked on. A moment that stretched into eternity as a block passed with no sign of Watson, and another.
By the third block, Watson was once again at his shoulder, silently offering the gun. Holmes accepted it without a word, smile, or sign of relief.
Out loud, that is.