Mycroft doesn't only use the surveillance cameras to keep his brother safe. He also uses them for his hobby: to study the Consulting Detective and it's chosen mate, the Ex-Army Doctor, in their natural habitat. He's nearly finished his thesis on the subject.
Obviously cracky, but if you manage a non-cracky version I applaud you.
Johnlock not necessary, but not opposed. I will, however, offer a singularity if Mycroft comments on how their actual mating habbits seem very rare or that they take an awful long time to reach that stage or whatever.