If they own anything you came up with while working for them, then they would own the drawings\business plan etc. Any future documentation derived from that original documentation would be a derivative work and there by be infringing. On the patent side of the fence you would have to have an idea capable of being patented, the wording of the employment contract would have to cover that angle as well. Having those two in place means they very well could come after you for the idea.
