Do a test.
If you have the handset point it at the radio as it is. If radio functions as expected try the next step.
Cover the rectangular box to blank it. Try the handset again. If it fails then the rectangle is a sensor. If the radio works, blank off the sensor in the radio faceplate and try again. There is a chance that there are two sensors, the rectangle being a piggyback to enhance use.
You could also then clear the rectangle bit and repeat the test.
You have thus sussed out which sensor(s) work.
Have you had a look behind that rectangle? Is there any plug/socket/wiring to it?