He worried.
Constantly.
He told himself it was actually a good thing his cameras couldn't find his brother, because if they couldn't, then Moriarty's men sure as hell wouldn't find him either.
But still, he worried.
Lying to Lestrade, Ms. Hudson and even to John had been a breeze. Pretending to be heartbroken had been disturbingly easy. Actually letting his little brother go off on his own, hunting down God knows what, was the hardest thing he'd ever done.
There was the occasional glimpse, a flash of a smile sent to his ever-present cameras. He took that as an 'I'm fine, you're an idiot.' And there were the less-than-frequent calls for assistance, information or 'back-off-I've-got-this-under-control.'
It was the hardest thing he'd ever done.
The times where he could tell his brother not to, where a simple 'no, Sherlock, be careful, that dog will bite,' was enough to keep him safe, were long gone.
Not that that had worked very well back then, actually. Mycroft almost smiled at the memory of a five-year-old bunch of curls and blue eyes, enthusiastically and fearlessly running towards Mr. Fischer's rather mean and ugly Labrador. His mother had not been pleased.
He breathed in, deep and even breaths, knuckles turning white around his umbrella. He couldn't come to understand why this hurt so much.