Hello Stonyly,
Would you mind sharing the processor configuration you are using? The latest Surface Pro i5 models are utilizing Intel's 7th generation processors which are designed to run cooler, therefore, Microsoft removed fans from the design to provide a lighter device with extended battery life.
The i7 models can still warm up quite a bit, especially under heavy usage, and still require a fan. Do you hear the fan turn on? How do you use your device?
The magnesium body is designed to absorb heat and cool down quickly which is why you are noticing fast cool down after you have stopped using the device. Users have reported normal operating temperatures reaching up to 75 degrees Celsius.
In my experience, some users are alarmed by the Surface due to the fact they can feel the temperature vs using older plastic OEM brands with larger fans and more space between you and the processor. The heat you are feeling is most likely normal and should not harm your device, however, if you are local to a Microsoft Store you can visit for a diagnostic checkup and determine if the machine is running at above normal operating temperatures.
Please let me know if you have additional questions!