When you point the Thermal Leak Detector at a window, it will measure a combination of the infrared light coming through the window and the infrared light coming from the surface of the window so the measurement will not be accurate.

To measure the temperature of a window, or other reflective object, put a large piece of masking tape on the glass, and wait a few minutes for the tape temperature to match the glass temperature. Then point the Leak Detector at the tape.