It's implied that the mole was either Laurence Dominic or Claire Saunders, but both don't really make sense as Alpha would. There was an episode where Topher round a piece of tech that would allow someone to screw with the imprints, it was never explained how long it had been sitting there. I think, for all intents and purposes, the mole storyline has been dropped, or is meant to be one of those mysterious to allow fans to find thier own answers and reasons for.
The possible list of people (whether its them or not)
Laurence Dominic (There was an episode that cantered him and the whole spy thing, some of his comments supports and denies this).
Claire Saunders (She has the skills to, but seems somewhat unlikely that she'd have heard of Ballard let alone act as the mole)
Ivy (She's outright said she had the skill to)
Alpha (The skill, the motive, but was there an opportunity to?)