"Gotta see a man about a dog," he says, and somehow it's a relief that John isn't nearly as perceptive as his brother Mycroft.
The truth is that he understands the sentiment only too well; this incident digging up a handful of childhood memories he never truly deleted from his mind palace, and it takes more effort than usual to keep himself aloof and detached.
Of course they couldn't bring themselves to put down the dog; not if he looked at them like poor Redbeard had done, mutely begging them to spare his life.
But this, John can never know.