A person who on the outside appears normal and acts normal. But in secret is a class A (Heroin / Crack / Meth etc) drug addict. The phrase comes from the R. Whites Lemonade television advertisment song of the 1970s / 80s as seen on British Television. In the advert a man is seen creeping downstairs at night in his pyjamas to drink lemonade from the fridge. Surprisingly there are quite a lot of Secret Lemonade Drinkers who are pillars of the community and occupy respectable professions - Doctors, Solicitors, Judges, Police Inspectors etc.
Yeah, did you hear about whats-his-name, apparently he is a "Secret Lemonade Drinker"! You wouldnt believe it of a vicar would you?