Their thermal capability is very well known. Shouldn't it catch the human body temperature with ease, at least during the night time, considering the surroundings would be cooler ?
I would think that medium altitude long endurance (MALE) drones could prove useful. They should be equipped with thermal cameras, synthetic aperture radar which can penetrate foliage and other optical sensors. Flying time 24 to 48 hrs will enable continuous surveillance. The SAR will enable the drone to work even in the night or in cloudy conditions.
Using such unmanned platforms will be cost effective.
If it can be armed with an LDP and LGB's or laser guided missiles it would allow the operator to take out targets once identified. Or may be two drones working in tandem - one only for surveillance and target identification and another armed drone for acting on the data collected by the ISR drone and taking out targets. The Laser Designation Pod will illuminate the target and help the missile or bomb to home in and explode. In an ideal scenario.
There will be a lot of variables involved and everything may not go 100% as planned. But if the chances of success are even 50% then the option can be explored. The armed drone will be sent only if an target can be identified.
Will not be easy. Perhaps like looking for a needle in a haystack. But we could get lucky.
Also monitoring the electro magnetic spectrum can prove useful. If the terrorists are using satellite phones, then maybe the location of the phones can be tracked down. Doubt whether normal mobile phones will be used. Also it is assumed that the terrorists communications will be in encrypted form. If underground optical fibre cables are being used for communication then it suggests the involvement of some official agencies of hostile governments.