Résumé : Surveillance technologies have become ubiquitous, yet they are still shrouded in secrecy. This article assesses the relevance of the systematic use of freedom of information requests as a data collection method, to document the evolution of the surveillance infrastructure in French-speaking Belgium. The article discusses the internal factors (specific to the research team) and external factors (specific to the authorities approached) likely to influence the level of administrative transparency. Our results reveal significant differences depending on the type of administrative entity, the relevant legal standards, the political color and size of the municipality. They also suggest that this method may prove relevant as a complement to case studies on surveillance, provided that substantial human resources are available. This article thus contributes to the debate on the use of freedom of information requests as an original data collection method for social science research purposes, particularly in the context of public policy sectors where the culture of secrecy is relatively widespread.