Model of the microwave?
It really is a microwave moisture sensor. See here
https://www.electro-tech-online.com...icles5Cwhy_microwave_moisture_measurement.pdf for a more complex explanation.
https://naldc.nal.usda.gov/download/8917/PDF
soem info I have from a service manual:
SENSOR COOKING CONDITION
Using the SENSOR COOK, SENSOR REHEAT or POPCORN
function, the foods are cooked without figuring time,
power level or quantity. When the oven senses enough
steam from the food, it relays the information to its microprocessor
which will calculate the remaining cooking time
and power level needed for best results. When the food is
cooked, water vapor is developed. The sensor “senses” the
vapor and its resistance increases gradually. When the
resistance reaches the value set according to the menu,
supplementary cooking is started. The time of supplementary
cooking is determined by experiment with each food
category and inputted into the LSI.
It seems there are two resistive elements, one measure moisture and the other humidity. (Microwave model Dacor DCM24)