Motion detection does not work through windows reliably. Arlo cameras use two different kinds of motion detection, and both types are inhibited by glass, especially windows.
Passive infrared, the type of motion detection used by Arlo Ultra, Arlo Wire-Free, Arlo Pro Wire-Free, and Arlo Go cameras, detects changes in heat. Most glass used in homes today is insulated, limiting the movement of heat through the glass.
Frame-by-frame comparison, the type of motion detection used by Arlo Ultra, Arlo Q, Arlo Q Plus, and Arlo Baby, sometimes detects false positives due to moving reflections in glass surfaces.
If you are attempting to detect motion outside, we recommend placing your Arlo Wire-Free, Arlo Pro Wire-Free, or Arlo Go camera outdoors.