Recent years have seen a new and welcome transparency as to the legal framework governing many of the state’s surveillance activities in the United Kingdom. The decline of secrecy as to law, however, is only one side of the coin. There remains a significant secrecy as to fact: not only when and where the state’s capacities are deployed, but also what those capacities are. Taking as an example the situation of ‘IMSI catchers’ – devices which permit the detection of mobile phones and interception of its communications – this contribution suggest that this degree of factual secrecy makes it difficult (perhaps impossible) to secure accountability for the conduct of state surveillance, notwithstanding the new era of legal clarity in which we find ourselves.