Plans of Mice and Men
221B Baker Street is a good place for a mouse.
It's not that the human lodgers are unaware of the fact that they have acquired mice. Despite Basil and Dawson's insistence that the parlor of the Master, as Basil called him, remain off-limits to the rank-and-file tenants, a man of Holmes's talents could hardly fail to notice an infestation as large as that of Holmestead. Holmes noticed, as he noticed everything, and Basil had heard him expound upon the subject many times to Watson. He described methods for getting rid of mice at great length -- chemical formulas for rat poison and mechanisms for traps so clever that they could easily catch all but the most astute of mice. Basil does not begrudge Mr Holmes these speeches, of course; mice and men, he knows, are enemies in many respects. But the traps and poisons Sherlock speaks of never appear. As intriguing as the theoretical problem of mouse-removal might be, his interest remains theoretical. Actual mice are inconsequential -- they do not hinder his ability to work, and so he ignores them. Watson, somewhat more inclined to the practical, often says that he'll inform Mrs. Hudson or something of that nature, but he never seems to remember.
And so Basil comes and goes as he pleases and sits under the Master's chair every night with his notebook, happily unnoticed. And for a man to ignore a mouse in his own household -- why, that is the kindest thing a rodent can expect.