Presented herein are systems and methods directed to a multispectral absorption-based imaging approach that provides for rapid and accurate detection, localization, and quantification of gas emission from within a site to be monitored. 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 a plurality of sampled locations along the reflector installment, for example by imaging multiple sampled locations at a time and/or scanning an instantaneous field of view (ifov) of the optical sensor. Lines-of-sight from the optical sensor to sampled 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.