There does not need to be a blind spot in the sweep of the cameras for Sherlock Holmes to pass through. All there needs to be is an element of timing. The cameras cover the whole grounds, yes, but not all at once. And he can find the gaps.

He circles around until he has inspected every side of the house, choosing the best spot to make his approach, watching the cameras from well out of range until their rhythm is imprinted on his mind. Guards, too. It will not, he determines, be easy, but it will be possible. Third floor, end of the hall, on the right. There are four suitable points of entry on the route he plans—one balcony, two windows, a skylight—and if he finds them all fitted with alarms, he estimates it will be possible to slip past one of them.

The thing about a house is that if you put too much security on it, it can start to resemble a fortress, and some people find that uncomfortable. From what he saw when he was inside, the Chandlers' precautions are not as extensive as the Starks'. Windows and doors not rigged with sliding steel panels for emergencies. Camera coverage inside the house less than complete. Yes, this is doable, if he makes no mistakes.