Sensing by Proxy: Occupancy Detection Based on Indoor CO2 Concentration

We introduce a sensing-by-proxy paradigm that infers latent occupancy from CO2 measurements using constitutive models capturing spatial and physical system structure. A link model relates proxy signals to unknown human emission via a coupled PDE–ODE system with data-driven parameters exhibiting stability and robustness across experiments. Field evaluations using both a CO2 pump and controlled occupancy show superior accuracy to ML baselines for estimating room occupancy, enabling occupancy-aware HVAC, lighting, and services.