Yeah, about that, there was an artist named Ken Penders who worked on Archie's Sonic the Hedgehog comics. He sued Archie and Sega over the characters he created for the Sonic stories. Apparently work-for-hire due diligence was not done on Archie and Sega's part, because rather than squash him like a bug they settled out of court saying in effect: "go away, and take your stupid characters with you".

So apparently, companies do have to go through a few hoops to make their claim on their employees' IP airtight.