Couldn't one argue that, by instructing the employee to throw the item in the trash, the employer was relinquishing its ownership of the item?
On the other hand, if he ate the expired sandwich in a manner visible to customers, that's different.
It used to be common practice that at the start of every shift the restaraunt would serve the wait staff an entire dinner off the menu. We all sat together. After shift we were entitled to a free drink as long as we sat at the bar.
At least at the places where I would even want to eat or work!