Rex Plus is the "Electronic Watchdog That Never Sleeps." You put this little box behind a partition (door, wall, curtain) and it detects movement on the other side, playing one of three sounds in response: a barking dog (described as "ferocious attack dog"), a barking dog followed by a siren, or rainforest noise (bird, frog, running water). When the motion stops, the sound stops, but it resumes if the motion does. You can also choose to have Rex play a continuous rainforest sound (perhaps to soothe nerves after the hubbub above). Rex is sold for about $80 at various online stores.
We tested the device by sneaking up on it from behind a wooden door, a plaster wall, and a steel door.
Rex is no mastiff, but it could be useful. The device responded to motion behind wood and plaster, from up to 30 feet away in some cases, although it ignored motion behind steel and around faraway corners. (You might need more than one device to protect separate entrances, and you'll need to adjust range and volume for the best effect.) The barking sounds real but isn't the deep "woof" of a huge dog, and there's no growling. Rex might deter a burglar looking for an easy target, our tester reports, "but someone who knows you have the Hope diamond in your house will probably not be deterred by the sound of this dog."