I suspect capacitive coupling to a buried cable would do it.
With the plaster all wet and conductive, the capacitance to
something like a light switch drop with both wires live would
be higher than normal.

When working in the loft on some earthed metalwork, which meant
laying on my side to access it, I noticed I was getting a strong
50Hz hum feeling when brushing against it. Checked with a neon
screwdriver, and it lit dimly. Then I realised I was proping
myself up on one arm, which was laying along a lighting cable.
Lift arm off cable, and the 50Hz hum feeling and lit neon both
stopped. Cable was in perfect condition -- it was just capacitive
coupling, helped by me being rather moist in a hot loft.

However, you should check this out thoroughly, remembering the
death of the MP's daughter just recently after people had noticed
something similar with a metal cutlery rack, which was in fact
live.

--
Andrew Gabriel