Usually, it's a clause in the employment contract saying that the company claims ownership of anything you create while under their employment, under the theory

Usually, it's a clause in the employment contract saying that the company claims ownership of anything you create while under their employment, under the theory that a salary covers more than just the time you're in the office. Maybe combine that with a company policy against releasing internally-produced source code, or something? My employer has something similar to that contract clause, but I've never heard of them actually invoking it.