You might be comfortable taking that risk yourself, but if you misrepresent your FOSS contributions as your own copyright you impose that risk on third parties. Tricking people into infringing your employer's copyright is asshole behavior.
I'd be surprised if there was any actual burden on the upstream maintainer to care whether I was on my lunch break or whether I was on the clock when I made the fix.
https://news.ycombinator.com/item?id=21771144
Although in this particular case, I tend to agree with Igor as he was employed as a system administrator not a software developer so it's unlikely that there were any real contractual constraints imposed on him in relation to copyright or invention transfer.