What sensor used in all non-contact body temperature guns?
Of course there is no answer to &quot;what sensor is used in all temperature guns&quot; but my guess would be thermopile sensors.
They are quite inexpensive and there are ASICs around which handle everything. Reading the actual measurement, making the cold junction adjustment and putting the value on a seven segment display.
Everything together can be had for less than 10 bucks.
Considering your description of the sensor a thermopile is even more likely. You have four pins: ThermoPile +, ThermoPile -, TemperatureResistance +, TemperatureResistance -
The TP pins will give you the thermopile voltage (millivolts) depending on where you point it at it and on the temperature of the sensor itself it can be positive or negative voltage.
The TR pins will give you access to some form of temperature sensor, depending on the accuracy it can be anything from a very low cost NTC to a very high cost platinum sensor.

Here is an example of a sensor which I got a quote for several years ago:
Datasheet
In a very low cost infrared thermometer they actually cut off the TR pins and used a different temperature sensor. I guess they used a different (cheaper?) sensor than the ASIC was intended to see and attached a fitting temperature sensor to it.

For an accurate measurement, the price will rise though, you'd want a more focused optic to have a rather small opening angle and a small measurement point.
Then you need some form of calibration. As the decision between fever or healthy cannot be done with a sensor which has +/- 2 K and some sensors don't get better than that even with calibration.
Then nice to haves are added like a laser pointer to indicate where you are pointing at and so on.