Sensor in Apple Logo at the Back of iMac
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.