Mycroft wonders why she always avoids tall buildings. He doesn't find out until they go to Seattle, and Mycroft insists on treating her to dinner at the top of the Space Needle.
It would have been lovely, if she hadn't been too busy trying not to have a panic attack.
Mycroft notices her discomfort (of course), because Mycroft notices everything. They leave early, and Mycroft apologizes by buying her ice cream and taking her for a walk in the park.
It's only later - when they're back at the hotel - that she realizes Mycroft has some problematic habits of his own.
An accidental slip of her hand that turns the lights off. Mycroft screams.
That's how she learns out that Mycroft Holmes (The Ice Man, The British Government), is afraid of the dark.
She doesn't tell. They pretend like nothing has happened. Nothing changes, except the addition of a small, red, rocket ship-shaped night light in Mycroft's office.
It turns out that old habits don't just die hard. They're immortal.