Sensor in Apple Logo at the Back of iMac I am cleaning and preferrably washing the iMac aluminium casing. I have removed all the hardware from the iMac except the sensor placed at the back side where Apple logo is. I would like to know what exactly is that sensor and if it could be removed? 

I tried exhaustive search but it appears that all roads lead to dead end. So I would like to ask the type of the sensor and can it be removed? My guess is that it is ambient light sensor but I am not sure.
You do not list which model of iMac you're cleaning, but in general the alu iMac models do not have a sensor in the back logo. Perhaps you're talking about the WiFi antenna that is located there on some models - you could ofcourse argue that an antenna is a type of sensor.

If you're actually talking about the front Apple logo, then older iMacs had an IR-sensor there for taking input from the Apple Remote.