... when you finally give up looking for something and just bloody-well buy a replacement, the formerly lost items will turn up, but not before you've used the replacement, so you cannot return it.
In other news, it turns out my 'good set' of bike lights were in the glove-box. Of course - why didn't I think to look there?
The other (less well known) truth is that the P*nct*re Fairy is simply one of a large clan of magic entities whose entire function is hiding things with cloaking spells.
The Allen Key Fairy, Spanner Fairy, and in this case the lights fairy. Your lights were in the twilight that exists between the Fairy world and ours
The Wallet Fairy (se note) then reports the purchase to their colleague who replaces the lights back into our world
(Wallet Fairy
You go for a night out with £10 in your pocket and spend £5 on beer you should have £5 left, but the Wallet Fairy ensures that there is always less than you expect and you only have £3)