How do SPST neon-indicator-lamp wall switches power their neon indicator?
If the live side is interrupted by the switch being opened, what is actually powering the neon lamp in the switch? Am I correct in assuming the 24VAC measured at the outlet box when the switch is open is a residual voltage from powering neon lamp?

It sounds as though the neon indicator is wired as a switch-open indicator.



simulate this circuit &ndash; Schematic created using CircuitLab
Figure 1. Probable wiring.
A neon lamp won't conduct until the test voltage exceeds the neon's breakdown voltage - typically about 60 V.
If the indicator is wired like this the indication will be ambiguous. It won't light when the the power is on or if the GFCI is tripped or if nothing is plugged in and switched on so that the neon can have a neutral return path.