Can a company enforce a noncompete that I was never aware of?

Can a company legally enforce a non compete contract without my knowledge or signature of such an agreement? I still work for the original company but I lost an opportunity at a competitor's business when a third party placement agency informed me of a non compete contract the hiring company had with my company during the hiring process. I was recruited for this position by a placement agency which determined that I was a good fit for the job which is a different position then the one I currently have. Also I am just a non exempt laborer.

When I attempted to get a copy of any paperwork showing that I signed a non compete the HR lady told me it did not matter whether or not I signed one I am covered under it.

I also attempted to get a waiver from the plant manager but he assured me that the president would not allow anyone to have a waiver.