How did you go Relle?

I spoke to my boss about it, he thinks the most likely explanation is something called 'enamel pearls'. The outer layer of teeth is made up of enamel. Sometimes when the teeth are forming some extra enamel can form, and will make it's way through the gum at some point. It will likely erupt and then exfoliate (fall out) sometime after. It is apparently not uncommon, so your MCHN should know about it. Not much to be done, just wait for the fragment to come out. I hope I explained that clearly enough for you

It could be something else, but he seemed to think that this was the most likely explanation. Thank you, because this was not something I'd heard of, so it's nice to learn something new!

Anyway, let me know how it turned out, I'm interested to know!