Neon Rainbow Always Near Top?

As you say, if it was all of them that would be worrying but just one isn't as much concern. It may mean that just the one fish is affected with something but the rest of them seem OK. Lumps and spots would have been the first thing I'd ask you but you've beaten me to it, and she's fine in that respect. [For other readers, dwarf rainbowfish are prone to these, and they are a bad sign].
It could be that this particular fish does have some internal problem (since there's nothing to see on the outside) but there are so many things that could be so it's hard to suggest a treatment. I would keep an eye on her for now. If you have a quarantine tank, perhaps move her into there. And if you do have a QT you could try something like eSHa 2000. I would not treat the whole main tank yet though.