Thermal camera shows the surface temperature of the object. Thermal leakage or change in material thermal conductivity can be detected with thermal imaging. There are some physics that need to the addressed.
The emissivity of the object affects greatly to the measured temperature. Typically matt surface objects (timber, concrete etc.) have emissivity around 0.95 which means that they absorb and emit 95% of the inbound heat radiation. Uneven or coarse surface or just dust on the surface will have an effect on the emissivity value. The shiny metal surface is opposite of matt surface in regards of emissivity. Shiny objects usually have emissivity around 0.05, so they act like a mirror to thermal radiation and rather reflect temperature of the surrounding objects than their own. This means that they show in thermal image false temperature. Also some materials exhibit change in the emissivity value with the angle of view. Concave surfaces with moderate emissivity values can be problematic because the surface radiates to itself resulting higher temperature value on thermal image. So one should be wary with those shiny and/or concave objects. This translates that advanced skillset is needed to correctly interpret the thermal images.
The surface temperature of an object is affected by thermal conductivity. If we consider outer wall of the building, the increase in moisture content will alter thermal conductivity of the wall materials, and thus overall thermal conductivity. The thermal insulation layer of the wall contributes most to the thermal conductivity and thermal conductivity is also most affected by moisture increase. The change in the surface temperature of the buildings outer wall detected with thermal camera is most likely caused by moisture increase in the thermal insulation layer.
At this point a reader might question that they thought that thermal camera is used for finding air leakage on the building outer structures. Usually air leakage is consequence of structural movement (displacement, contraction/expansion) caused by excess moisture in the structures. So we are back on the moisture related problem. Of course on new or under 2 year old buildings there is air leakage caused by construction phase and settling of the structures. The most problems in buildings occurs during rest of the life cycle. To US locating and detecting moisture damage without demolition of structures (possibly in vain) is of special interest. And finding damages in their minor state is both tecnically and economically wise.
Successfull thermal imaging requires temperature difference over object. This sets restrictions on when thermal imaging can be performed. But for example after a cold night outer structures of the building are cooled and possible damages are more easily to be found. In principle from inside of the building damages are seen as cooler areas and from outside warmer areas. After the warm and sunny day outer structures are warm and colder surfaces can be indication of excess moisture evaporating.
Thermal imaging can be perfomed from the inside or the outside of the building, taking into account the restrictions mentioned above. With thermal imaging possible damages can be pinpointed and allocate repair resources where it is most needed.