Thermal signature can be generated in two ways: artificially and naturally.
An artificially-generated signature is usually heat generated by mechanical means, such as an engine. A Thermal Imaging (TI) device will register generated heat from both distance and altitude. TI imagery is quite clever and is utilised by both military and civilian organisations.
In the world of tactical reconnaissance, TI imagery can indicate the presence of, or recent presence of enemy forces. Even an empty airfield can show signs of recently departed aircraft due to the heat generated by their engines: the heated tarmac showing up as a TI signature quite clearly. As I said, quite clever. The same can be said for vehicles. There's not much point camouflaging one's 'Lanny' and then leaving the engine running. Not only is it dangerous from a Health and Safety POV (as well as adding to global warming), it's sure to attract a bombardment from one's opponent - no matter how much hessian & scrim you use.
Allegedly when TI was first brought out some bright spark had the idea of putting a nice warm thing in the middle of bales of hay so that it looked to a helicopter pilot exactly like a tank, and then hiding in some convenient bushes with a BFO rocket launcher. When the steely-eyed angel of death came along to destroy the 'tank' - boom!
Natural thermal signature is what happens when you run. You will glow like a Windscale beach towel on a TI. This is particularly problematic if you're running from someone or something with TI assets, e.g. a police helicopter. Running in to the nearest wooded area to hide in the bushes is a pointless exercise, as the scuffer's TI will spot you easily - vectoring the furry crocodiles and associated brain on a chain directly on to your position with uncanny precision.
- Immerse yourself in freezing water, thus lowering your TI signature. This is risky, as you'll have to breathe. So unless you routinely carry a snorkel, you're fucked. You also risk hypothermia.
- Wrap yourself in a thermal blanket, thus reflecting your heat back in (and not out like a beacon). Routinely carry one of these just in case the flying pigs pitch up looking for your sorry arrse. Unfortunately, this too has its drawbacks, as you'll sweat like a rapist and the doggies will find you.
- Try and mix with a crowd. You might have to go back in to the pub you've trashed - the incident that started all this business in the first place. Don't carry out FIPAC drills in remote or isolated pubs. They'll hunt you down with ease.
- Bury yourself. Whilst covering oneself up with earth reduces one's signature, it's not quite enough to escape the clutches of the law. Remember too that even dead bodies generate heat by the natural chemical process of decomposition, and it didn't stop them finding the one in my back garden.
- Give yourself up (see above). There is some honour and kudos in surrendering oneself to a hovering aircraft. It worked for the Iraqis in '91, and it's an infinitely better option than having a dog chewing on your testicles.