A dive suit will probably have a lower than ambient temperature because of its insulating qualities, but there will still be ain infrared signature that will depend upon its emissivity as well as its outside temperature. No matter what that emissivity is, it is likely to produce a net infrared brightness (radiance) in the detection bad that differs from the background, and therefore will be noticed by a motion detector.
There is no way to suppress both emission and reflection of a surface. The sum of the emissivity and reflectivity is 1.0. Even if you could supress both, an moving object with no infrared output would look like a moving black hole in the background, and could easily be detected with a motion-detection algorithm
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