From Chapter 5 of SIGN, I quote (Watson regarding Bartholomew Sholto's grounds): "It looks as though all the moles in England had been let loose in it. I have seen something of the sort on the side of a hill near Ballarat, where the prospectors had been at work."
"And from the same cause," said Holmes.
Yet another of Doyle's interesting discrepancies...in BOSC, which was set post-marriage, the word Ballarat is mentioned as a major connection in the case...and yet we hear no more about the fact that Watson obviously had been there. Holmes in SIGN is not surprised to hear this; obviously he knows Watson was in Australia at some point, but that is never brought up in BOSC. Address this plot hole however you please.

A/N: My muse is awake again, but still very grumpy about being repeatedly shaken and asked for ideas. I'll just keep feeding him sugar; that usually works for me. :)
This is one of my more AUish prompt answers...

"Can you explain something to me, Sherlock?"

Holmes met his older brother's gaze across a table in the Stranger's Room. He was visiting Mycroft at the Diogenes club for want of anything else to do. He had no cases, and Watson was away taking care of some business, leaving him listless, alone, and bored out of his mind. He had not resorted to the drug yet today, for his body craved movement or action of some sort, and in any case Mrs. Hudson was going through one of her annual bouts of cleaning and he decided that not being in her way was the best course of action. So he had gone to see Mycroft, who seemed to pick up on the fact that he was acting as a sort of diversion for his younger brother, but had declined to comment on the fact. They had sat mostly in silence until now.

"What is it you need explained?"

"I am just curious--Is Doctor Watson extremely prone to distraction at inopportune moments, or is he just an imbecile?"

Holmes narrowed his eyes. "Mycroft, we would be in the midst of a heated argument right now if we did not both know that you know he is neither. Now what are you driving at?"

"I've just been reading this." Mycroft pulled out an old copy of the Strand magazine. Sherlock internally rolled his eyes. "It struck me," said Mycroft, "how completely absurd it was that there were these references to Australia--Ballarat, no less--where the good Doctor spent time in his boyhood according to his account of the matter of the Sign of Four. And yet he was completely at a loss throughout the matter. More so, the two of you did not once speak with each other about Ballarat, though it was clearly of importance in the case."

The younger Holmes leaned back in his chair, outwardly showing no sign of expression, inwardly dearly wishing his older brother had kept his mouth shut just this once.

"It just seems highly improbable, Sherlock."

Holmes casually lit a cigarette, not meeting his brother's eyes. "It is highly improbable."

"Then what is the explanation?"

"Do you really need to know?"

"It is suspicious enough that you are not telling me, Sherlock."

There was no answer.

"I could always ask him myself, you know."

"He would not tell you."

"Then your explanation will have to suffice, brother."

Sherlock glared at his inconveniently perceptive elder brother. "I do not see why it is of such great importance, Mycroft."

"I am curious as to the truth of the matter."

Holmes sat very still for several moments. "The truth, brother?"

The elder Holmes nodded.

"The truth is that he solved it before I did."

Mycroft had not been expecting that, though later he wondered why. "What?" he ejaculated, less dignified than he had meant.

"He solved it before I did," Sherlock repeated.

"...Oh." Mycroft cursed himself for his sudden bout of inarticulateness. After all, what had he been expecting? It made perfect sense.

Sherlock seemed to pick up on his thoughts. "It's the way he writes his stories," he explained. "He makes me out to be the great detective, and himself to be the mere spectator. You never expect him to make connections on his own. That's why you're so surprised."

"Well... it makes sense, anyway." Mycroft leaned forward as much as he was able. "Care to tell me what really happened?"

My comment to Lestrade that the description I gave should lead him to the killer was genuine. I had no doubt that finding such a person should present no great problem--it was less helpful than knowing the man's exact identity, of course, but I was sure it would suffice.

I was, however, extremely irritated that I could not find anything more definite.

Watson, conscious of my moods as always, allowed me to stew and ponder in silence for some time. Finally I could take it no longer. "I am so close, Watson," I burst out. "I just cannot seem to come any closer. I know there must have been some connection to his son's marriage. I know it, I can feel it. I just can't place the connection. I cannot find a motive. Turner was against it, the man's son was against it--but reason enough to kill a man, that I cannot find." I began to pace. "It all boils down to whoever he was calling to that day, when his son mistook his call of "cooee" as a signal to him. If we can learn who that is, Watson, I am sure it would tell us who the guilty party is at the same time."

Watson seemed to pause for a moment, then spoke, but with a note of reluctance in his voice. "I've been thinking about that, Holmes."

"And what have you come up with?" I asked, almost out of habit. These conversations had become common between us, a familiar routine we had both grown to know and love. Today, however, he sounded uncertain.

"I--well, the man had lived in Australia, Holmes," he began.

"He did, Watson."

"Well, I've been thinking about what the boy said--his father was calling "Cooee" to this unknown person. That's a particular call that originated in Australia--he taught it to his son and they used it to call each other--but this man didn't know his son was there, so he must have been calling to someone else--someone to whom the call would mean something--someone who had been in Australia as well."

I stood perfectly still and stared open mouthed as Watson pushed on, speaking faster now, as if making sure he said the thoughts aloud before they disappeared. "His son said his dying words were some sort of reference to a rat--when I read about it, I didn't think anything of it, but I kept on hearing it mentioned--and it started to sound familiar. And then I realised there were lots of places in Australia that end with "-arat"--Such as Ballarat. And if he was making a reference to Ballarat when he died, it would have to do with his killer, wouldn't it? And the person who fits that description is..."

"Turner," I breathed, staring at him. "My God, Watson."

"It's hardly unreasonable for him to have noticed it before you, Sherlock," said Mycroft. "After all, he did spend some time in Australia--Many elements of the man's life would have been familiar to him in a way that they would not be to you. Not to mention you first saw the fragment "arat" in print as "a rat," and continued to think of it as two words."

"Of course that is true, Mycroft. But I should have seen it! I had all the information I needed, I just failed to put it together."

"So tell me," said Mycroft, "Why did he not write up the true account?"

Holmes ground his back teeth and stared at the tabletop. "I don't know," he admitted finally. "He said it was for the readers--they were reading the adventures of Sherlock Holmes, and read his accounts for the mysteries--they would want to see the case played out and solved by me."

"Rubbish."

"Of course. But he wouldn't give me any other answer. Probably he was worried it would bruise my pride."

"Well. It would have."

The younger Holmes glowered. "Perhaps you are right," he growled through gritted teeth. "But even so--he reasoned it all out remarkably well, and he rarely gives himself any sort of credit in these accounts of his. I would never have prevented him from revealing the truth of the matter."

"No, you wouldn't have." Mycroft studied his brother. "But he changed the story all the same. Perhaps he did it because he knew you would not have stopped him if he had chosen to do otherwise."

"Must you always speak in riddles, brother?"

"It's not a riddle, Sherlock. Just give it some thought. He's your friend."

Holmes gave it some thought. Watson would never explain his reasons, of this he was certain. But perhaps there was something to what his brother said. My faithful Boswell--will I ever reach your limits?

There were some cases, he decided, that could remain unsolved.

A/N: Watson really doesn't give himself enough credit--so I gave it for him.