1. A method for detecting effluent from a target under observation, said method comprising: at an image processing device comprising a processor and memory: receiving a first image frame and a second image frame of a scene captured in a field of view of an image capture device; identifying a first region in the second image frame with a parameter variation value satisfying a first threshold criteria, the parameter variation value defining a rate of change in a scene parameter between the first image frame and the second image frame; and generating an output in response to the presence of the first region in the second image frame, wherein the first threshold criteria defines a rate of change in scene temperature between the first image frame and the second image frame, and wherein the first region corresponds to a first area of the second image frame in which the parameter variation value meets or exceeds a maximum rate of change of scene temperature between the first image frame and the second image frame.