Maybe I'm showing my age, but when I was in college, I had an internship at a company called "Detection Systems" with the guy who invented and patented a low-cost PIR sensor based on ink. In those days ultrasonic sensors were popular, and PIR was just starting.
I'm not exactly sure why ultrasonic isn't used as much anymore, but I can guess a few reasons. They can have false alarms on occasion, for good range, the transmitter and receiver have to be spaced apart a bit, and its possible some pets people keep were bothered by the high-frequency sound. Also, I'm not sure this was ever proven, but for a time, with powerful transmitters, there were some concern for negative health effects, at least with long-term exposure.
I have that Leviton OSSMT-GDW ultrasonic/PIR motion switch in my garage, but my wife always yells at me because it leaves the light on for long time after we are no longer in the garage.
I don't mean to be really down on occupancy detectors. I guess I'm reacting to the years when I started in home automation, and had the dream that many have of motion detectors controlling all the lights for you. Using them to turn on lights is problematic because of the delay usually when used with an OMNI, sensors and lighting, all which adds a few seconds.
Occupancy sensors can be used to turn light off, but you need to be very generous on the time. I do this with a two hour timeout in our family room, and still sometimes the lights go out when we are sitting there. Unfortunately, occupancy detectors have limitations in real life.
I have heard of people detecting Bluetooth from your phone, or an RFID tag to detect the presence of someone, but it can get expensive.
If you use a sensor, just learn what motion it detects. RF and ultrasonic detect things moving toward or away from the sensor. PIR detects things moving across the sensor, but only across in one axis, so be careful how you mount it. And test it a bit before you permanently drill holes to install it. Try it before you fully commit.