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 now 39 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: