If you are doing it in a company office, using company property, during hours the company expects you to be working, do not expect to use "well, I'm salaried, so it doesn't matter" as a legal defense and win.
Company property is a completely different issue. Whether you do it during business hours or not is irrelevant.
If you’re salaried, you’re always on company time, so whether the company owns it or not is up to the laws of the state and the agreements you signed when hired.
What if you did most of your project outside the company, but only very small parts of it on company property? Can they still claim complete ownership?