Reflectivity. The metal housing is reflecting the cooler temperatures behind the thermal camera and including it in the imaging.

Relatable similar occurance: I was working on an offshore oil platform once using a thermal gun to try to identify a leaking valve and found out pretty quickly that the polished steel gasket between the valve and pipe would reflect the temperature of open sky. I thought I found a leak when the gun registered a chilly 32F but was actually at like 100F on contact.