You can't sense temperature remotely except via heat radiation which is normally only from the
very surface layer. However I suspect if you get such a leak the whole section along which the leaked
air flows will cool, probably spottable with a thermal camera. You can lean a lot with a thermal camera.
Note that metal surfaces dont radiate heat at all well, almost any other material radiates nicely. If the
pipe insulation is clad in aluminium foil it will be harder to see anything. It often is clad this way to
further reduce heat loss.