ALBF, mission narrow down:
1/ Extension lead with two pin or three pin plug, in other words: earth wire nearby? The same for this now famous reading light. Your multimeter (hint: father's day 16th of June this year) should be able to detect in it's high resistance range if there is something wrong. Or try another extension lead.
2/About this reading light: old school Mr. Edison bulb or a modern led thingy? Including perhaps a sort of power line filter (unlikely).
3/ Just in case this reading light - or power supply of a laptop - has a filter on board it could be a case of a (half) defective capacitor inside
. Provoking a little surge current to earth when switching off. When your mains sine wave (two upper traces)
is riding high or low, the effect will be more pronounced. Happens at random, depends on the timing of disconnection. Only very experienced qualified electricians are able to switch off when the phase is exactly in between top and bottom of the sine wave. Solution #1: confiscate reading light and your son's laptop. Solution #2: hire a qualified electrician, 24/7.
4/ Spot in the ceiling including earth wire? Check and double check.
5/ Bathroom and garden wiring: always happy to bother you with nuisance triggering of RCD devices.