A few years ago I worked for a software company and we discussed several hundred different ideas for software, but never did any kind of testing or prototyping, we simply discussed the ideas. In other words, there's no identifiable embodiment of any of the ideas. I left the company years ago and would like to build one of the ideas that we discussed while I worked there. Am I prohibited from building the idea on my own because the idea was conceived while I worked there? I'm sure I signed some kind of NDA when I was hired.
This is an Australian perspective; other jurisdictions will differ.
The only IP protection for ideas is if they are patented; embodying the ideas in a tangible form is a pre-requisite for acquiring a patent. You can check if this idea has been patented because if it has then it is a matter of public record.
However, there is another method of protecting ideas; keeping them secret. It is highly likely that the information that you gained as an employee is confidential. You say you signed a non-disclosure agreement; you will be bound by whatever that says. Even if you hadn't, confidentiality is an implied term of any employment contract; an explicit term can reduce its coverage or make explicit what is and is not confidential.
For breach of confidence to be established:
- The information must be confidential (it probably is);
- The circumstances of the disclosure to you are such that you know or should know it is confidential (they probably were);
- Your disclosure of the information must be unauthorised (it probably is) and to the detriment of the company (if you make money off it then they could have and that is to their detriment) and you do not have a public interest defence (you don't).
- An injunction to restrain disclosure;
- An accounting of profits (i.e. what you make, they take);
- An order for delivery up or destruction;
- an order for seizing evidence.
There is no statutory limit on how long you must maintain the confidence; presumably until disclosure will not cause detriment.
- Ask for authorisation: "Remember that idea we discussed years ago? If you are not doing anything with it do you mind if I have a go?" You may get a simple "Sure, go ahead." in which case document it (send them an email confirming your conversation) or you may get a "No" (that's the risk with this option) or you may get pages and pages of license agreement (deal with that on its merits).
- Do it anyway - they may not sue, if they do they will have a hard time proving that you are using their ideas in the absence of contemporaneous evidence.