The surface temperature of a human is about 90 degrees Fahrenheit. If the background temperatures are significantly cooler (or hotter) than 90 degrees, a human target will stand out much more on the TI display. This is why TIs tend to be more helpful in night searches rather than day searches. During the daytime, the sun will heat up the surrounding environment, bringing the temperatures of the background objects closer to the 90-degree mark. This will make it more difficult to discern a human heat signature.
Also, the type of thermal imager can affect detection range as well. While many of the detectors available today have similar sensitivities, there are a wide range of performance characteristics. The one of most concern is field of view (FOV). This defines how wide an area the TI scans and displays on the viewing screen.
A wide FOV (50 degrees or more) portrays more information on the display; however, it makes objects appear smaller than they really are and makes distances appear farther. A narrow FOV (10-20 degrees) displays items at more lifelike size and distance, but you scan a smaller area. There is always a trade off in performance. Since the narrower FOV displays objects closer to their true size and distance, these generally give a greater detection range. For example, a 50 degree FOV will have effective human detection out to 70 or 80 meters. An 11 degree FOV will have effective human detection to roughly 300 meters.
Note that most fire service TIs have FOVs of 50 degrees or greater. Most law enforcement TIs have FOVs of 10-20 degrees. So while a fire service TI can help in a search, and may be better than nothing, its performance in an outside search will be less effective than a narrow FOV imager.
A search for lost or missing people is rarely just a law enforcement function. However, if we are involved as police officers, we can use our thermal imagers to help us with those searches. The imagers will be more effective in poor visibility than during the heat of the day, but this is not a firm rule.
And the nice part about a search and rescue operation is that everyone wants you there and wants you to succeed. That breaks our normal dichotomy of life as officers...and might even make us as popular as firemen.