Polarization and change in capacitance

The concept of electric field polarization is a convenient explanation for describing the phenomenon of change in capacitance between the positive and negative plates of a coplanar capacitor. Any object which has certain dielectric properties when placed under the influence of external electric field, it gets polarized. This concept has also been illustrated in Fig. 1. This polarized object causes the redistribution of charges on positive and negative plates of the co-planar capacitor which can now accomodate more charges, not shown in Fig. 1. This requirement of extra charges which results in change in capacitance of the plates according to C = Q / V relationship is fulfilled by the source which could be a static cell or a dynamic signal generator. 

Similarly human body or wood or paper can also alter the capacitance of the plates which could be detected by the circuit and if the electrodes (positive and negative plates of the coplanar capacitor) are not properly designed then it could also trigger false alarms.