I don't doubt you, but my experience is different.

Every company I've worked for (mostly F500s) has had an onboarding document you sign in which you're notified that everything inside the company is confidential by default. It doesn't expire.

Most of these companies also had annual training (the usual corporate policy refresher stuff) and this was mentioned during that as well.

I've worked on some very hush-hush projects. In my experience, NDAs were for people outside the corporation...partners, vendors, etc. Everyone who was employed by the corporation was under an NDA by default. I don't think I've ever signed a subsequent NDA for something my employer was working on, because I was already under one.

And even if they didn't do all this formally...it's a reasonable expectation. I would never think I'm allowed to discuss anything that wasn't publicly announced with outsiders.