Other leak detection methods sense materials INDIRECTLY. That is, they detect an event caused by a leak instead of sensing the product being leaked.
A float switch assembly, for example, only detects a movement of the component's electrical contacts. The float switch assembly's electrical contacts are, in turn, being activated by a movement of the float. In the final analysis, the float MAY or MAY NOT be responding to a product release.
The same would be true of inventory reconciliation, tank gauging, and pressure decay methods. Factors other than a product release may be responsible for an "indication of a leak".
Two-wire, impedance measuring, cable type systems (TDR - Time Domain Reflectometry), measure impedance changes on a coaxial cable. When either water or fuel enters the braiding of these sensors, the impedance on the cable changes, the control panel recognizes this impedance change, and attempts to interpret the meaning of the change. If it is successful, the control panel then attempts to determine the type of material (water or fuel) being released and where the event is occurring. Considering the significant differences between the dielectric constants of water and fuel, it is a formidable challenge.