There are quite a few vehicles whose doors are capable of being controlled by the computer - above 5mph, the doors automatically lock. When put in park, the doors unlock. I'd consider this to be plausible.
I don't know much about the wireless signal sent by keys to remotely unlock a vehicle - but I do recall that a more expensive remote control that I owned at one point was capable of recording directly from other remotes. So, it also seems conceivable that someone could simply wait near any parking lot, record people unlocking their vehicles, then play that back the next day (assuming they park in that area daily.)
Computers control most everything in modern cars. Entry-level Korean cars were probably the last holdouts for some of the more 'old-school' stuff. No chipped key, no keyless entry, ect. May have only had a PCM. But even a Kia Rio now has a BCM with some basic immobilizer/anti-theft functionality, I'm pretty sure.
Any car with power locks is certainly going to be operating them via transistor. Even if a car is available without keyless entry, it makes financial sense for the automaker to not make two different systems. So the unlock button on the door of your non-keyless model is interacting with the same computer that would be triggering the door locks as a result of a remote unlock. Or because vehicle speed has hit a certain threshold or whatnot.
I think Rudeguy addressed the feasibility of this pretty well. You're just not gonna 'brute force' your way into your average car using an RF remote. I'm not sure about the tech employed in smart keys...I had thought that was still a similar RF system, but maybe not? Bluetooth would make sense, I guess.
For anyone that hasn't used a smart key, the important part is that they generally have a proximity unlock feature- you walk up to the car with the key fob in your pocket, touch a button on the door handle (or just touch the handle itself), and the car unlocks. I assume that your touch triggers the car to say 'hey, a person...checking for key fob...' but I don't know how the fob is detected. I would think it would have to see a request from the car and then respond actively...a constant passive signal doesn't seem very feasible.
Unless there's a newer exploit based around the smart keys, I still just think this is bullshit. The only feasible method would be capturing the signal from a remote and then somehow using that to broadcast your own valid unlock request. And that seems very, very difficult. People with the necessary brainpower are probably not the ones interested in stealing stuff out of your car.