I'm using iStat Pro Dashboard widget to get live system data of my mid-2010 27" iMac.
Under temperature readings it lists Ambient. Where the sensor is approximately located?
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As per the manual and take apart guide provided by Apple to Apple Certified Mac Technicians: the ambient temp sensor is attached to the CPU blower (or fan).
I found this on iFixit below the page at Step 8 you will see the temperature sensor in orange
EDIT : that's the temperature sensor from the LCD
At Step 12 they mention the sensors but don't really highlight them.