Can a WiFi sensor track mobile devices from upstairs and downstairs?

Yes, it is possible. However, WiFi signal strength typically weakens once there is obstruction (e.g. floor, glass). From a few trials we performed, we were able to pick up signals from downstairs but only for a split second. Thus, the error resulting from these false signals is not significant.

The solution to this problem is simple. You may employ more WiFi sensors at closer distances and reduce the range of each WiFi sensor so that signals from other floors will never be picked up or reduced to an insignificant level.