NB forwarding does NOT include "spam" email... i have all my Gmail accounts funnel into one and i check the spam buckets of all, every 4 weeks (otherwise Google turfs em). i usually find a few (rather important) false positives in that monthly sweep.

Further note that gMail filters at every step, eg this includes a downstream "archive" account. So there are false positives coming from a "known" [single source] good account and of already vetted emails...

i do wish there was a way to forward everything ... where everything meant everything ... filtering optional.