Maybe. Hairline cracks in the intake or exhaust ports would be really hard to spot. A head gasket leak would be seen when they did the cam swap, for sure, but it could also be that one of the head gaskets started leaking as a consequence of the work being done because the heads have to come off. That said, a leaking head gasket is extremely rare on one of these. And the fact that the puff of smoke is becoming less frequent suggests a different cause because a head gasket leak usually gets worse quickly.
Since the radiator had to come out for the work, it seems more likely that your coolant loss is the result of one of the radiator hose connections leaking a little bit or some other coolant hose connection being not quite tight. You might take a flashlight and look around the radiator and hose connections to see if you can spot any wetness anywhere. Coolant also smells faintly sweet when it's hot, so a leak may be accompanied by a slight odor.