I think the fact that “check your spam” is so widely understood is a testament to false positives in the inbox being an issue for Gmail.

Ideally this phrase would not be so widely known and understood if Gmail didn’t have so many false positives going to the spam folder. I’d imagine that, as a performance metric, the Gmail team would consider false positives to be a metric for improvement, not a metric of pride.

GGP's point is that in some circumstances, Gmail will simply disappear emails, with no trace, instead of quarantining them.

Who has time for that when your spam folder is full of hundreds of junk messages per day?

It is not.