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.
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.
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.
I'm not aware of any use of liquid nitrogen to keep general infrastructure cool but it sounds like you're referring to the gas tanks that keep wiring under a positive pressure to keep moisture out.
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.
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.