Any #brompton experts here?

I was folding my (P line) Brompton the other day and heard a small "ping" sound. Then I saw this small, black, anodized, Phillips-head screw on the ground - but I can't find where it fell off from!

Anyone have any ideas?

The bike still works fine, at the moment: it gets me around the streets of Pisa quickly and comfortably, easily on the train even when crowded, even up&down the 4km long, 350m high hill near my house -- a reliable companion!