Also helps never to mix company and private email (separate email accounts) and code (two computers). If the side-project becomes any serious, e.g. you want to register a trademark or you have to change your tax status because it brings in extra money, try to get approval in writing, can be as simple as an email.
If the company tries to claim "yes, it' in the contract, but we don't enforce it" you could point to the Facebook movie (The Social Network), where lack of clarity/paper-trail has cost somebody 10s of millions.
