Welcome.
You've probably already found & read at least some of the thread that's stickied in the WI forum titled "
New to open carrying in WI? What you should know"
If you haven't found Act 35 already, I have a link in my sig to a PDF, & a couple FAQs.
And while I agree with protias' interpretation that an encased firearm is out of reach, I don't know that a cop would. There's what the law says in plain English, there's what the cops do on the street, there's what the lawyers argue in court, & there's what judges decide.
As for your original question about the case having to be in plain sight, no law requires that, but a WI appeals court decided the concealed weapons law requires as its elements that:
a) there was a 'weapon' on your person or within reach
b) you knew it was there
c) it was hidden
(See the very bottom of the left-hand colum on pg. 3 here:
http://legis.wisconsin.gov/statutes/Stat0941.pdf)
Some cops will try to claim that since it's "hidden" in a case that means it's concealed if it's within reach. (Not saying I agree, just pointing out what has happened... in fact, it was one charge discussed in the Brookfield incident.)
But putting it under the seat, or under a jacket on the back seat, would give more weight to their claim that it was concealed. (Unless you can only get to the back seat by getting out & flipping your seat forward. Then I think it'd be treated like the trunk, but I'm neither a cop nor a lawyer, nor do I play one on TV.)
In fact, look at the top of the right-hand column on that same page, State v. Walls. He was convicted for 'concealing' for having a pistol on the passenger seat with nothing over it but air, because the cop couldn't see the pistol from outside the car.
(But if it had been drugs instead of a pistol they would have claimed it was in plain sight, so they could seize it.) :banghead: :cuss:
There is a request in to the AG to clarify the interaction between the new transport law (saying OC of a pistol in a car is OK) & the Walls ruling (saying that unless it can be seen from outside the car it's concealed).