It's getting harder to be a terrorist, and much of that grief comes from cheaper thermal imagers, and more powerful pattern recognition software. One example of this is the ACT 1 (Access Counter IED Technology) thermal imager. This device can examine individuals approaching a checkpoint, or simply moving near a base. The software examines the thermal image of the individuals, and, using a library of data on how explosives or weapons show up under clothing, determine with a high degree of accuracy, who is armed, or wearing a bomb.
ACT I is small, it looks like a large pair of binoculars sitting on a tripod. The entire systems weighs less than twenty pounds and can be operational in less than 15 minutes. Systems like this are getting smaller, more powerful and cheaper. A knowledgeable terrorist can do things to deceive it, but not all your terrorists are that smart.
In Iraq and Afghanistan, similar systems, used as part of base defenses, to track suspicious activities, or assist in the pursuit of enemy forces. The U.S. Army is developing a new generation of remote sensors that uses similar technology. These can be air dropped, or placed by troops on the ground, to provide more monitoring capability in remote areas.
The ability of the police or military to store vast quantities of this data also allows for suspicious activity to be sought out after a crime has occurred. Criminals, and terrorists, usually scout out a location before carrying out a crime. Advances in pattern recognition and statistical analysis software helps investigators find things that are not easily noticed at a glance.