The Arduino build preprocessing cannot locate the sensor source and header files if they are located in another sketch directory. The files needs to be moved to another directory if shared (as a library).
--Sketchbook
---NodeIntercom
----NodeIntercom.ino
---MasterNode
----MasterNode.ino
---libraries
----Sensor
-----Sensor.h
-----Sensor.cpp
The Sensor header is included with
#include <Sensor.h>
in the sketches that needs to use it.