I have no expertise in leak detection systems but suspect that a flammability meter (explosion meter) may be useful to detect hazardous conditions as a result of a leak.  There is a whole body of information
which should be available in the safety literature concerning the operation of heat transfer fluids above their flash point and the appropriate safety measures which are required.  I recommend researching this and working with the oil supplier as well to find a solution which prevents these hazardous situations from occurring or going undetected if they do.