PHOTOS: A mysterious self-driving Toyota with a stealthy paint job was spotted cruising around San Francisco (NVDA, TM)

The Toyota Highlander was packed with sensors and other gear, but it's not clear which company is behind this self-driving car prototype.

null  (Luc Oreskovic)

Self-driving cars are running wild in the streets of San Francisco.

Residents of the city are likely to encounter a robo-car prototype at any moment. Uber, GM, and Google spinoff Waymo are the companies behind the most frequently sighted autonomous cars. There have also been scattered reports of vehicles presumed to be tied to Apple.

But that's just a small sampling of all the self-driving cars motoring around. There are now39 companies testing autonomous vehicles on California roads. Among them: Samsung, Mercedes Benz, Baidu, and AutoX.

On Sunday, after spotting what seemed to be a new kind of self-driving vehicle, we followed it. The customized Toyota Highlander (model year 2014 or later, according to resident car expert Bryan Logan) was brimming with sensors and other gear. And it sported a custom paint job that seemed designed to keep it under the radar (at least, as much as that's possible for an autonomous vehicle).

Check out the pictures below for a closer look at this mysterious vehicle:

We spotted the vehicle on Sunday afternoon in the Bernal Heights neighborhood of San Francisco, which is near the Mission.

We spotted the vehicle on Sunday afternoon in the Bernal Heights neighborhood of San Francisco, which is near the Mission.  (Luc Oreskovic)

We hit the gas and pulled up close behind so we could get a better look.

We hit the gas and pulled up close behind so we could get a better look.  (Luc Oreskovic)

We noticed a few curious things right away. The vehicle appears to be a Toyota Highlander, yet the Toyota insignia has been removed from the oval-shaped space above the license plate. The entire body of the vehicle was painted a matte black that gave the vehicle an intriguing and stealthy profile.

Notice the four, puck-like gizmos.

Notice the four, puck-like gizmos.  (Luc Oreskovic)

These appear to be LiDARs — the laser sensors used to create high-resolution maps and to let cars "see" and navigate their surroundings autonomously. There were four more of these LiDARs mounted on the front of the car, for a total of 8.

The top of the car had some other interesting gear, including what appeared to be multiple video cameras and other strange-looking sensors.

The top of the car had some other interesting gear, including what appeared to be multiple video cameras and other strange-looking sensors.  (Luc Oreskovic)

Here's another look at the vehicle's collection of roof-mounted hardware.

Here's another look at the vehicle's collection of roof-mounted hardware.  (Luc Oreskovic)

The black box-like items on the right and left each appeared to have a camera lens tucked inside.

We got a better look at the vehicle as it turned left (perhaps to try to evade us).

We got a better look at the vehicle as it turned left (perhaps to try to evade us).  (Luc Oreskovic)

This gives you a better look at the hardware in the front of the vehicle.

We pulled up alongside the SUV and confirmed there was a person in the passenger seat, but he ignored our requests to roll down his window and answer our questions.

So whose vehicle is it?

Nvidia CEO Jenn-Hsun Huang at CES 2016  (Matt Weinberger/Business Insider)

Toyota does not have a permit to test self-driving cars in California. But the automaker struck a partnership in May with graphics chip maker Nvidia to develop self-driving cars. And Nvidia is one of the 39 companies registeredwith the California Department of Motor Vehicles to test autonomous cars on public roads.

We showed Nvidia our pictures and asked if the SUV belonged to the company. The chip maker declined to comment. We also asked Toyota, but that company did not immediately return a request for comment.