I have one, but I don't even know where it is. In the years I've been doing this, I've only been asked once.
The other day I went out with MikeOregon. Good thing he had his on him because the lady who wanted approached us stated we couldn't do it in any city park, thats what she was told.
It was funny, because he started explaining to her that yes, it is allowed, and she cut him off saying, "No, No, you can have a permit for a detector, but not to hunt in the city parks!"
Mike explained to her that yes, as a matter of fact you can, and here is the permit to prove it.
She asked who'd you get the permit from?
He showed her- the city parks department.
It was all very civil and cordial, the way it should be. He even offered that if she still wanted us to leave, we'd be more than happy to leave anyways.
She evidently made up her mind that she was misinformed, she said no problem. I chimed in that we always leave a place better than we found it, and we proceeded on detecting with no problems.
The key to resolving these issues is staying calm and courteous, not getting into heated debates or flat out arguing about it.
So that makes twice now, in about 10 years of detecting here in Eugene. Never have I been approached anywhere outside of the city.