[thaumasiotes]

> or is it called a false positive because it is a positive for which you made the error of calling it negative?

Not to pick on the non-nativeness of the problem here, but that's not really a way you can use "false". I'd be a lot more comfortable calling an underlying positive that tested negative an "unidentified positive" or a "misdiagnosed positive" [this one really is ambiguous in exactly the way you suggest] or anything else that suggested that the positive was there and an error occurred in noticing it, as opposed to suggesting that the positive wasn't there in the first place.

So, it's a fair complaint for non-native speakers, but you just can't choose all your terminology to meet their needs. :/