If you get plenty of infrared radiation, you could look at modifying a photographic camera with infrablue filters. I personally use a 580 nm infrared pass filter and just remove the IR cut filter from within. It helps if you choose the right kind of camera to do this with, because some are pretty complicated. maxmax.com sells those cameras already modded.
If the kind of IR is too near, then you need a far infrared camera like a FLIR (Tau range). That will for sure pick up heat and visualize differences of even 1 degree.
You can also start with smartphones and "personal" FLIR cameras:
A lower cost solution is the IR-Blue: http://www.rhworkshop.com/products/ir-blue-dm
That is a thermopile sensor array, but only has 16x4 resolution . What determines your success rate there is whether you can get close enough.
Using an FLIR camera would work well here. however FLIR cameras are super expensive and small ones to mount on a drone even more so.
How quickly do these voids form and move?
if they do not form quickly and move slowly then you could purchase satellite imagery at a fraction of the cost of doing this by drone.
If they do form and move quickly then maybe you could just hire someone to walk in front of the machine with one of those laser temperature readers? or alternatively photograph from the ground using a handheld FLIR (there are companies here in sa with qualified technitians that you can hire to identify hotspots on things like bearings/power lines etc)
Finally, the use of drones here in ZA is somewhat illegal at present.