This depends on the type of target and the type of context. For instance you can use TIR images to detect objects not very well illuminated. On the contrary if you are looking for metallic targets concealed in vegatation (i.e. tank), you can use SAR images.

We may use multi band imagery technique

Some type of microwave or millimeter wave method may be of use depending on the specifics of the problem.   Millimeter is used in airport security applications for example while microwave frequencies are employed for foliage and ground penetrating problems.