Pressing "Park" should unlock the doors. Trying to go forward does release the parking brake. In my experience, any time the car thinks you are being stupid, it beeps. And usually, that means, "Step on the brake, stupid!". I've even had it beep at me when my foot was on the brake, and I'm shifting into R. It still goes into R, so I think it's being a bit pedantic, like I tried to do it "too soon". It almost sounds like you had one of those rare instances when the computer gets confused and stops responding to your inputs. But I don't know. I thought I knew all the modes for this thing, but that "seatbelts fastened" item was a new one for me. So, you never know what extra factor might be in play.
Powering it off would be the wild card, though. It unlocks the doors, and usually clears all the confusion in the computer. Then again, if it's really confused, it would refuse to turn off. There has been the occasional report of that, too.
And there goes the first capacity bar! At 24,000 mi on 9/9/2013.
Second bar at 30,500 mi on 2/7/2015.