I had this issue today where both fans in my rMBP spun up to full speed and would not stop. I checked the processes in activity monitor and CPU usage was very low so that wasn’t it. I live in Ireland so ambient temperature wasn’t the culprit either!
The problem persisted through a restart, which was worrying but a manual sleep and resume fixed it. To do this click the apple button, choose sleep, wait for it to turn off and then press a key to wake it up.