Where's the missing piece?

There's a story here. Some friends who'd recently stayed at our house in Aberaeron left us a present. Knowing that Mrs GM loves a good jigsaw they had found a jigsaw case in a charity shop and bought it for her. What they didn't realise  was that it also contained a jigsaw (without the original picture as a guide). Mrs GM has been working at it and this afternoon she, with a small amount of help from me, finished it. Surprisingly, it was complete, except that is for one single piece! Look closely and you'll see the gap. Oh well, it is a great device that can be taken out at a moment's notice with all work intact.