Here are a few examples I'm personally aware of...
AT&T developed the yacc parser generator, and lex lexical analyzer generator, as part of Unix. You were only supposed to get copies of the source if you'd gotten a Unix source license... but somebody swiped a copy off the desk off the desk of a person who shall remain anonymous here sometime around 1980. (It wasn't me, and I'm not sure he'd want his name bruited about.) The source started floating around, people ported them to the IBM PC, yadda yadda. I got copies from an outfit in Austin that was selling "source code for nifty programs" floppy disks around 1986.
Back around 1990, Microsoft had kind of a reputation for this, although I'm not sure it was exactly corporate policy. In addition to the Stac case Tangurena mentioned, a consulting company that had previously done work for Apple to port QuickTime to Windows re-used some of the proprietary QuickTime sources in a project for Intel and Microsoft to accelerate MS's Video for Windows. Apple wound up getting a stop-ship injunction against Video for Windows. It's certainly not clear to outsiders whether anybody at Intel and/or Microsoft knew about this theft.
This doesn't really happen very much, though, with US companies - the risks are far too high. For instance, I was a contractor at the now-defunct database vendor Informix when they were in the middle of a benchmark battle with Oracle, and Informix kept winning. Oracle hired away one of Informix's core database engineers, and he showed up to the job the first day with a hard drive full of the Informix sources, thinking they'd welcome him with open arms. They welcomed him, all right - with a security team to escort him to the police station. They also called Informix security to come over and retrieve the unexamined hard drive.