Conant will boil that is the reason asked  not open the cap at least 10-15 minutes after your drive. Even after 15 minutes, use some clothes and just unwind little bit and let the coolant will flow down,  is the safest  way to open radiator cap.

Thermostat controls the fan and once the engine overheats up the coolant will heat and the thermostat is designed to kick the fan on once it reaches certain temperature level.. If the fan is not running you may see the heat indicator in your dashboard  going up normally it stays half way or less,  never let it go over 60-70% (red zone) it will damage your car head gasket and it will cost you $500 and over to replace it, still you need to fix the thermostat..

So, while you drive this car, make sure the heat indicator is not too high. Did you ask you mechanic what did he do on your car? Did he change any thing? What was the reason ? Most of the time they keep your car inside, they jack up and leave for hours and hours, do little things and charge for bucks.

I had overheated problem and it was due thermostat which cost 7-10 dollars and takes about half, someone changed the radiator and finally the head gasket gone, paid about 450, That mechanic did not change the thermostat nor advised for it, again blown the head gasket, finally dumped the car.