If you're not already wearing a tinfoil hat, it may be a good time to start: a pair of engineers based out of the University of Michigan have figured out a way to create a light-powered camera sensor that's only a millimeter in size: small enough to be practically invisible to a casual observer.
According to a paper published in IEEE Electron Device Letters by Euisik Yoon and Sung-Yun Park, the new camera has the potential to not only be insanely small, but also, self sustaining, thanks to a solar panel placed directly behind the camera's image sensor, which is thin enough that light, in addition to what's needed to create an image, is able to pass right through it. This could provide the camera with all the power it needs to be able to continue to capture images, indefinitely. At a maximum of 15 frames per second, the images it captures aren't of the best quality, but they're more than adequate for creeping on an unsuspecting subject.
The good news is that, for the time being, the camera is nothing more than a proof-of-concept. In order for it to be deployed in the real world as a near-invisible surveillance device, someone a lot smarter than me will need to figure out how to store image data and transmit it using hardware that's just as discrete as the camera's image sensor and power source are.