If it was that important Apple should have been more careful than to let their engineer take it out and about. Given that people are prone to losing cell phones, how could they expect that not a single one would be lost?
Shit happens. No measure could have prevented one prototype from being lost eventually. And mobile phones have to be field tested before release.
There is a measure that could prevent a prototype from being stolen or sold to a blog that would take it apart bit by bit and post videos of it: it's called the law. I think they passed that a few hundred years ago.
There are ways to field test that don't involve people taking phones to bars. That is just carelessness. Sure, shit happens, but c'mon, a little responsibility might be in order on the part of the company with so much to lose. Law or no law, Apple should take more care with such prototypes.