Presented herein are systems and methods directed to a multispectral absorption-based imaging approach offering improved detection, localization, and quantification of gas emission. The imaging technology described herein utilizes an optical sensor and broadband illumination in combination with specialized reflector installments mounted about the site. The optical sensor detects light (e.g., reflected) from locations along the reflector installment. Lines-of-sight from the optical sensor to locations along the reflector installment sweep out an “optical curtain” partially enclosing and/or forming a boundary near various assets to be monitored. Optical absorption signatures from leaking gas crossing the optical curtain can be used to detect, localize, and obtain quantitative measures characterizing the leak. Measurements from reflector installments can be combined with measurements obtained via reflection of ambient light from background materials in a hybrid approach that expands monitoring capabilities and offers improvements in detection.