There is a silent trigger. If you look at the new Apple TV interface at the top you will see if there is a rental available. Over time the outline changes colors to eventually become red. A few hours after it is red the authorization will end.
You're thinking of movies and TV shows rented
directly on the Apple TV. The article is referring to movies and TV shows rented
in iTunes, which can now be streamed to the Apple TV. Different situation, and the Apple TV definitely provides no indication that you're looking at a rented item; it just sorts it amongst your normal purchased content.
The only clue that it's a rental is that if you look at your
playlists (which have to be specifically enabled in your Apple TV preferences to show up for anything other than music) there will be a "virtual" Smart Playlist labelled "Rentals" that includes all of your rented content. However, the same content also appears everywhere else in the UI with no indication that it's rented, nor any warning when you actually start playing it.
If you can watch it again then it is a bug, but if you can't watch it again then things are working as per normal, just silently.
To be fair, it
is a bug, just not a loophole that will let you keep watching a rented item. The bug is that iTunes never gets the updated expiry information, and will end up keeping a useless unplayable item cluttering up your library long after it should have expired -- the only way to get rid of it is to delete it manually, which of course requires you figuring out which rented items you've watched and are therefore ready to delete.
I've found with my new Apple TV that it often doesn't mark shows as watched after we have... watched them. These are just shows I've downloaded via Bit Torrent and put into iTunes.
That's also a bug. Right now the new Apple TV has more bugs than a bait store in terms of how it interacts with iTunes. You will find, however, that if you stop watching a movie or TV show within the last few minutes (generally around the last 10% of it), it
will correctly mark it as watched. I've had to get into the habit of ending my TV shows before they actually get to the end just to work around this annoying bug.
Other fun bugs include the fact that the TV show sorting is completely messed up (even leaving aside the fact that you now get one entry per season), and all playlists now sort alphabetically instead of preserving the sort order from iTunes.