I usually include, as one of the disclosed items, something along the lines of "other open source or business ideas I may come up with or have come up with on personal time and while using personal, non-company, property."
One of our employees did this too, and I took no issue with it. My guess is if you have an employer suing you for infringement based on work you did there, you have burned a bridge and have much bigger problems than just this lawsuit.
Edit: IANAL, this is not legal advice, etc.