Some materials attenuate Wi-Fi signals more than others. Metal and masonry attenuate microwaves more than wood or fiberglass or plastic. As a result, it is important not to install your sensor in a metal enclosure (known in this context as a "Faraday Cage") if you are using the sensor's internal antenna. In this circumstance, external antennas are a better choice, both for the sensor and for Wi-Fi or cellular backhaul.
Attenuation of Wi-Fi signal strength is measured in decibels: Low attenuation of Wi-Fi is in the vicinity of -50db; high attenuation of Wi-Fi reaches or exceeds -120db. The BlueFox solution uses the power of a mobile phone's Wi-Fi signal to estimate its distance from our sensor.
Many commercial buildings have metal inter-floor panels, explaining why it's so hard to see a Wi-Fi access point on the floor above or below. This isolation naturally limits the BlueFox detection zone to the floor on which the sensor is located. Similarly, if a building has concrete or masonry walls, Wi-Fi attenuation is high, also naturally isolating the inside of a building from its outside. If a building has wood frames, Wi-Fi will much more easily permeate the walls.
Here are some examples of different wall materials' impact on Wi-Fi signals.