peter wrote:
Maybe I dreamed this, but I'm sure I remember seeing advertised a
gadget in two parts which you could place either side of a surface to
show you where a drilled hole will exit on the blind side. In case I
haven't described that very well, you would have someone hold one half
of the tool at the point where you intend to drill, then you go round
the other side with the other part of the tool and move it around until
it indicates, by light or sound, the point where the drill is going to
exit. Anyone know what I'm talking about? Thanks

screwfix sell a thruwall locator
p/n 15433-76, page 499
sorry don't know how to munge their URLs