Ok everyone, I have a disappointing update but it does answer my question. I didn't actually get round to contacting Lego about this because I mentioned my discoververy to my mum first. She immediately said "Oh your father put that in there!" She told me a story about how he found the piece on the floor, thought he dropped it, asked my mum if it was Lego, then when she said no, he put it in the bag (I don't know why he didn't just put it in the bin, but okay). I was SURE the bag of spare parts was sealed, so this confused me even more. So I fished the bag out of the bin: you can see the big rip I made in it, but if you look closely, there's a teeny tiny rip at the top too... So I guess my dad just didn't want to spill the spare parts, so he made the tiniest opening he could to put the blue piece in there. The rip was very small and in the seam part of the bag, so I didn't see it and thought it was a sealed bag when I saw the piece in there! (So, SanderDeDycker's comment about the piece possibly getting into the bag after it was opened was 100% correct after all).
I'm marking MMM's answer as the accepted one because they found out what the piece actually is, which is very interesting :)
Sorry for the wild non-Lego related goose chase everyone, the story is kinda funny though (so I'll be keeping the piece)