I see these around NYC relatively frequently. Sometimes I generally also see gas canisters apparently slightly open in subways and street corners at the same time I see the gas detectors.<p>I have always assumed that whoever is responsible for placing the devices is releasing a benign (but distinct) gas and measuring it in several places downwind.<p>By doing so, the DHS could use the data they gather to triangulate the source of a chemical weapon, or to predict its most likely path of destruction for quick aid and response.