Now even if you didn’t use any company resources, most contracts provide at the very least the right for your company to use any IP you develop, even in your off time, even without any of their resources. Your contract in all likelihood would give them actual ownership of the IP.
You could choose not to tell them about this IP, and I believe you would be fine, as long as you never used or published it - even after termination of employment - because not being required to disclose the IP does not change ownership - it belongs to the company, even if the company does not know about it.
Source: going over job offers+contracts with a lawyer