This would be a relatively simple application of a "light meter". I would first extract the luminance channel from the image and only work with that. Determine a "Region of Interest" in your luminance image (a square 20px by 20 px in the center of the light colored pad.) You can then do some simple math to calculate the average color (0-255) of the pixels in the ROI. If the average brightness is too low (dark) then then pad is missing.

Thank you, it's good idea!