How accurate are the pressure test and block test for a blown head gasket?

I drive a 2012 Kia Soul (141,000 miles). I was on the freeway driving when suddenly white smoke shot out of the tailpipe. I took it to the dealership where they performed the pressure and block tests. It passed both. They also took it on a test drive. The car did not emit any white smoke and the engine did not overheat. However, the coolant level was low. Meaning whatever shot out of the tail pipe had to be coolant. However, they couldn't find any sign of a blown head gasket. They believe the problem was my radiator cap but couldn't explain the white smoke coming out the exhaust. I bought the car last week and assume they put on the wrong radiator cap. My question: is there anyway the head gasket could be blown despite passing both tests and how would you explain the white smoke coming out the exhaust?