Please note, IANAL, and for anything you should get proper legal counsel.
In the US, the courts frown very heavily on companies attempts to restrict their employees' moonlighting. It's seen as a restraint of trade, and indeed it is. You have a right to ply your trade to your benefit and you don't have to answer to your employer, tell your employer or anything of that sort. Even at that, if you want to keep your job, don't let them know either.
Companies are not prevented from requiring you to sign draconic sounding things, like "as of the date of hire, you have no IP and you'll sign over anything you invent to them while employed there, and after you leave you promise to forever after show them anything you might come up with so they can decide if it should be counted as something they paid you for while you worked there. I am speaking from personal experience now.
They will also try to get you to agree not to compete with them for either a certain amount of time or within a certain geographical area or both if you leave.
Remember, just because you agreed to it in writing does NOT mean you're bound to it. Contracts have to be enforceable to bind parties and almost everything above is unenforceable, that is, little of it holds up in court except stealing IP, in your case source code or perhaps customer lists.
I used to puke up all over the contracts companies wanted me to sign until my lawyer friend told me to go ahead and sign it because it's meaningless.. it will never be used against you in court. All these no-moonlighting, non-compete contracts are just there to intimidate you.
It's amusing that one person believes they can tell others that they have no right to engage in income producing activity except if they approve of it. In fact, since the days of Kings and serfs, such "rights" have been steadily eroding. You company has no such right.
If you're not using your employer's code, customer list, etc. etc. then you're in the clear. Of course you "own" what you've learned on every job; how could it be otherwise?
If your company has a no-moonlighting clause in their employment contract, and they find out, they may fire you, but they almost certainly won't cite the moonlighting clause as the reason since they'd be opening themselves up to a lawsuit with damages they know they'd lose.
If they find out and want to get rid of you, they'll claim your performance is not up to snuff and go through the whole dog and pony show of performance improvement programs and all that which is how they get rid of people they don't like for whatever reason.
Don't steal your employers' code. Don't work on their computer or on company time. Don't let your employer know you're doing what you're doing; it's none of their business anyway. There are ways to obscure the owner's identity in a company without breaking the law. Don't be nervous about doing what you're doing, but also don't tell anyone at all because that's the way they're most likely to find out.
Best of luck with your million dollar idea!