AC mains wire finder + live wire equivalent circuit
Your ground can be anywhere, it just a reference point. So you could draw your circuit like the first one. Remember air can be modeled as resistance and the metal in the wall and in your detector will form capacitance. 

One word of warning is the spice simulator may not like having a 1GΩ resistor in between it and the circuit, so the second circuit might be kinder on the solver. Its easier for the solver matrix to have more row zeroed out and makes it easier to determine a solution. 

Secondly it might be a better model to put the cap in parallel  with the 1GΩ resistance that simulates air, off the top of my head I'm not sure which would more closely model this situation, I'd have to run some numbers. 

Any two conductors with an electric field between them can be modeled as a capacitor. 

Any thing with current running through a material (even air) can be modleled as a resistor. 

Anyway hope that helps.







simulate this circuit &ndash; Schematic created using CircuitLab