Paiva et al. use biosensing to expose the body as both an evidential instrument and a source of methodological instability in urban fieldwork. Working with students in Warsaw and Cuiabá, the study examines electrodermal activity as a means of registering physiological reactions during movement through urban space. Its most productive finding is not that biodata objectively resolves subjective experience, but that physiological traces become meaningful only when reconnected to location, time, event and interpretation. Students repeatedly identified the need to couple biosensing with GPS, video and contextual observation because urban environments present simultaneous stimuli that resist simple causal attribution. Different climates, morphologies and participant characteristics further complicate standardisation. The conceptual operation therefore reverses technological determinism: sensing does not eliminate situated interpretation but intensifies the need for it. The paper connects urban geography to environmental psychology and mobile methods by showing that body–environment relations require multimodal evidence architectures in which physiological resolution and contextual explanation remain mutually constraining.