Material to record though so camera isn't visible? I am building an Arduino (https://www.arduino.cc/en/Main/ArduinoBoardUno) device that will record video based on a motion sensors, as well as other data in the environment. The thing is, I need to record from within a box to the outside, but the camera should not be visible. Any ideas or advice on how and what to use?
If quality of video is not so important to you, then you can use effect, which is used on public transport windows:


It is when you use lot of dots on the glass. From the outside it looks like semitransparent image. But from inside it will look like that:


With correct focus dots will be less visible on video.

And second option is to use half-silvered mirror.