Thermals present a heat based image of a person with no chance for identification of any facial features, and limited views of the body where parts overlap or certain postures are adopted. Therefore there are obvious limitations to the technology. However, despite not having a clear view of the person, thermals have their own body language to look for. Just the simple fact that there is a presence of the people in the area can be an issue, particularly at certain times at night when this is unexpected. Positioning at or being close to affected areas such as fence lines can raise immediate issues, and directions of movement to or away from fences or towards risk areas are critical factors to look for. Smoothness of movement, for example pausing at certain sections of the perimeter, or when a patrol moves past tells you about intentions. The volume of people can also be a critical factor in planning a response.
In additional there are particular body language activities which apply to thermals. For example, the technology is ideal for detecting behaviour such as crouching, crawling, clustering, or carrying. Movement speed including a response like scurrying away in response to threats is also distinctive. Objects being carried can sometimes be worked out by their own heat signature. In one of the mines I was training at, having infiltrated the mining area, a suspect was making off with a large section of plastic sheeting towards the fence, with the sheet heat signature trailing off behind him as he was dragging it.
There are obvious limitations to thermal cameras with any description or facial features besides build and height being impossible. However, by using natural body language and recognition of heat signatures, it is possible to use them for far more surveillance applications than may be expected. Thermals are not replacements for normal cameras; rather they have their own role in effective surveillance and are often capable to doing that exceptionally well.