user12708: 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?

user11523: 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.

user5987: Traffic cameras and security cameras shoot through a dark glass, so you could use something like that.  
I'm sure you've seen those black domes in stores - it's called a Pan/Tilt/Zoom (PTZ) dome.  You might look in the area of security camera accessories.

12708: <p>I am building an Arduino (<a href="https://www.arduino.cc/en/Main/ArduinoBoardUno" rel="nofollow">https://www.arduino.cc/en/Main/ArduinoBoardUno</a>) 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?</p>
 11523: <p>If quality of video is not so important to you, then you can use effect, which is used on public transport windows:
<a href="https://i.stack.imgur.com/iknr9.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/iknr9.jpg" alt="enter image description here"></a></p>

<p>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:
<a href="https://i.stack.imgur.com/9CEzx.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/9CEzx.jpg" alt="enter image description here"></a></p>

<p>With correct focus dots will be less visible on video.</p>

<p>And second option is to use half-silvered mirror.</p>
 5987: <p>Traffic cameras and security cameras shoot through a dark glass, so you could use something like that.  </p>

<p>I'm sure you've seen those black domes in stores - it's called a Pan/Tilt/Zoom (PTZ) dome.  You might look in the area of security camera accessories.</p>
