This just can't happen today, anymore. Everybody/-thing and their dog is bound at least by NDAs, if not worse by non-compete agreements.
The only exception I know of is Google's 20% program, but IIRC even this has been scaled down.
Is that an exception? Letting people work on whatever, for the company, is different than letting them work on whatever then walk off with it.
It wasn't a hard thing to do, and I get the impression that unless you're planning on directly competing with something they care about, they're pretty liberal with such releases.
If it becomes successful enough to launch into its own company, there is a huge disadvantage to not having the IP situation clarified up front. For one thing, investors get very concerned about even the remote possibility of someone else trying to claim copyright to your code. And once you are successful, it's not safe (or ethical, really) to just assume your [former] employer will never hear about it and connect the dots.