calling on the boys here to win me this argument. I normally would have just not worried if it wasn't this specific piece of a set I have managed to keep whole for 5 years.
I told her I'm gonna post this question and that the boys will know exactly what piece was lost to annoy me so much.
Hint: I live in a metric country and was about to do some work on the car.
Update: big thanks to everybody who replied, she has replaced my 10mm socket. Both of us had a good laugh at some of the replies and how fast they came in. Yeah the boiiis!!!!