Bob Minchin formulated on Saturday :
Find a miniature mains transformer - secondary voltage irrelevant. Wind a
turn or so of insulated wire through the odd gaps in the core and use this as
a current transformer with a neon across the 240v winding.
You might have to play with the number of turns but the neon will strike when
the lighting circuit draws current.

I have a similar arrangement in place, for PIR controlled lighting,
except instead of wiring a neon across the output I used a piezo
sounder. The output of the transformer just generates a few volts, only
enough for the sounder, so perfectly safe and the noise is difficult to
miss.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk