Yeah that's very tiny. Only thing i'd worry is if it were made of copper which I doubt and it's not the internals of a bulb itself,
so I wouldn't be worried about mercury or any weird chemicals (do they even use mercury?).
I would still keep an eye on things, but personally, if you can't find it, I think you should still be fine. In my old 29g it had a paperclip, a razor blade,
various beetle shells lost in it (bugs came in my room through an open window one night and died in the water), and never had any problems.
I know a little different from your scenario, but just trying to comfort you to know that not all things metal or alien are doomed to damage a tank. =]
Edit: Read your worry of copper in solder, but copper solder is used in pipe fitting & plumbing. Electrical solder is usually tin, or a lead/tin alloy.
AFAIK tin will oxidize into nothing over time in saltwater and should be fine. And most solder doesn't contain lead anymore.