237V seen across terminals of a dimmer switch when 'off'
There is always voltage across the circuit, supplied by the power company. By Kirchhoffs's Voltage Law, the sum of the voltages across the individual components must always equal the supply voltage. When the light is on, there is little or no voltage across the switch and all of the voltage appears across the light. But when the light is off, all of the voltage appears across the switch and there is no voltage across the light (and no current through it).