Almost 20 years ago I installed an electric shower in our house, with
a continuous run of 10 mm^2 T&E from the CU to the big switch in the
airing cupboard, then the same cable to the shower. A couple of years
ago when we got a combination boiler & a new mixer shower, I switched
off the breaker, disconnected the cable from the "load" end of the
switch, & removed the electric shower. The breaker has been off since
then.

Last weekend I was tiling part of the inside of the airing cupboard &
needed to move the switch out of the way. My volt stick said the
cable was live, but the neon on the switch didn't light up when I
switched it on. I went downstairs & checked that the breaker labelled
"SHOWER" was still off; the volt stick didn't beep at that end of the
cable. (There's no other 10 mm^2 T&E in the house, so it has to be
the right one.) I switched the breaker on & the cable beeped;
switched it off & it stopped.

Back at the top end, the cable still beeped but the neon wouldn't come
on. I very carefully opened the box & used a voltmeter on the switch
"supply" terminals: between 3 & 4 volts AC! Anyway, I carefully took
the box off the wall, put it back together, then tucked it & the cable
out of the way.

Where did the mysterious voltage come from, & why did the volt stick
only beep at the downstream end of the same cable? (I think the cable
does run physically parallel to a few other live ones in the floor of
the landing, but it's not coiled anywhere.)