It's on the back side of the clutch guard near the top of the primary. As others have pointed out, it's not the actual belt temp, it's the nearby air that the sensor is reading.
I've had the belt sensor alarm beep twice in 1200mi. Nice to know when you're running super hot, but honestly it's rare that this happens and you don't already have a feeling that you've been working the sled HARD.
Biggest issue with the turbo is engine temp. If you're running on hard pack for miles and/or in warm temps without a lot of cooling, they get HOT pretty quickly. But, we didn't buy these things to run as trail sleds.
So there is lots of variances with that I’m guessing, from snow load to ambient temps .. seen them five off a few alarms in over the hide snow where venting is not the best and hears quick.. but still a good tool