I had a mate who demoed his project to a large company years ago with a view to having them run with it and pay him royalties, flat fee or whatever.
They said they weren't interested and he dropped the idea.
A year or so later the exact product was put on the market by them.
So it happens.
Most people get you to sign an NDA, as to what that's really worth I have no idea, like most things it's worth what you can afford to pay the lawyers I suppose.
Rob