Yes, Esteban is correct in this point. Metal detector can sniff at some meters object size of a coin. A good system is off-resonance type at frequency between 60 to 160 Khz. Always, here, the problem is that this system need metal buried for some years for to be detectable. IR is very but VERY GOOD. No for to measure the temp, no, as a type of antenna that collect the phenomenom, an "electric field", or directly IR emission of metals. He post many types of pistols olds and news, so is a reality. I found a small object that EVER causes problem in electronic long distance detector, look in the video that is not very insistent for the size, but there are. The object I found (several times found the same type) causes a RESONANCE or microspark, DON'T KNOW, but Esteban discover this effect he call "effect E", at 5-7 meters of the detector-system. Download video here (take with cellular, sorry quality), more than 2 Mb:
http://www.sendspace.com/file/smrfnz
More later the object.