The only hit I could find by googling both "someone would have noticed" and "somebody would have noticed" (what's the difference, by the way... anyone? anybody?) and both phrases along with the word 'implausible' was this Twin Towers conspiracy site (which claims that someone would have noticed the odd explosions on the tape - so not quite what Alicorn was complaining about). This which makes the (I think resonable) point that someone would probably have noticed if Elizabeth I had ever been pregnant and given birth to a child. And this which explains that someone would have noticed if Paul had just suddenly made up the resurrection myth several months after the supposed resurrection without consulting any of the other apostles (which, to be fair, also seems pretty plausible to me).

I couldn't find any more uses of the exact phrase (I realise there are dozens of plausible paraphrasings, but I couldn't be bothered to think of all of them), so conclude that most of the time when people use this heuristic they are actually being reasonable (the person in the last link is very clearly wrong, but that doesn't make his reasoning invalid).