Cities are increasingly governed through data, yet the capital flows that fund and maintain ordinary public infrastructure often remain difficult to trace. This article conceptualises this blind spot as capital-flow data: the quantifiable flows through which infrastructure is funded, maintained, upgraded, and spatially distributed. Drawing on a purposive documentary study of advertising-funded public infrastructure in Scotland, centred on Glasgow City Council’s bus shelter concession and Strathclyde Partnership for Transport, the article examines how these flows become only partially knowable. It identifies four linked mechanisms: contractual relocation, which moves financial relations out of conventional budget lines; commercial confidentiality, which restricts disclosure under procurement and information law alike; proprietary valuation, which embeds value in market-specific measurement systems; and data friction, which raises the cost of reconstruction. The article further proposes a five-link framework of inscription, commensuration, consolidation, disclosure, and interpretability for locating where public knowability breaks down. It argues that produced opacity does not by itself demonstrate distributive injustice, but it constrains the capacity of publics and researchers to evaluate and contest how redistribution through infrastructure is organised.