I've been on both sides of this table quite a bit as an employee and as someone who hires a lot for like 20 years.
As an employer, what I like to look for when I ask for a GitHub profile is not that I'm seeing an insane prolific coder who will only spend time at work and on my business or "bleeds code". What I'm looking for is a signal that this person has the wherewithal to communicate code to a larger audience. Can they make their PR readable to someone else? Can they ship a project no matter how small that would make sense to an outsider? Can they teach other devs? Can they find things to abstract that someone else would find useful?
As for time, can I show you a couple projects of mine: https://github.com/n8/bust_rails_etags and https://github.com/n8/nectarine
Remove the boilerplate and they are both ~30 lines of code. https://github.com/n8/bust_rails_etags/blob/master/lib/bust_... and https://github.com/n8/nectarine/blob/master/lib/nectarine.rb
Neither are very popular or show off any code wizardry. These things aren't impressive in the least. You might even argue they are dumb and useless. Fine! But both projects have been great conversation pieces when I'm talking to folks trying to hire me.
I'm showing (at least trying to show) those same things I'm looking for as a boss: I look for things to abstract, I take the time to communicate with clarity, I teach other devs, etc.
And as for an organization who says: "Absolutely NO public code of yours on Github or you're fired." First, who are these orgs? There should be a shitlist of companies who wipe out their employees Github profiles. But... this seems to be a lot more rare of a case, and I've worked in a lot of places.
Yes, there's plenty worried about their IP being leaked, but there's plenty of code to write that teaches a concept that isn't near a company's IP. But if you're really worried about upsetting a boss, I would take a crack at talking to someone (a boss, hr person, etc.) who has the authority to take a peak at putting out a project or two like I describe. Since these aren't mind blowing things, but tiny little utilities or ways of showing off a non-proprietary concept, it's very likely you'll be met with yesses. Even that talk of convincing your boss to "YES" about publishing something sounds like a great story to share with a future employer ;)
So I agree, there are folks using open source poorly as a signal, but for those out there with the complaint that "My organization won't let me. Or I don't have time to contribute the amazing things I know I can contribute.", the hurdle isn't as big as you think. A lot of us employers aren't looking for amazing, gigantic, magic contributions to the world. Just a couple handfuls of code to show how you think and communicate with others.