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The inner lining of the brake hose can deteriorate over time and a "flap" develops inside the hose. If the loose end of the flap is in the direction of the caliper it will flatten itself against the wall of the hose when the brake fluid is flowing outward as normally happens when you step on the pedal. However, it will impede the return fluid flow when you release the pedal inasmuch as the flap is now perpendicular to and obstructing the flow in the direction the fluid is attempting to travel. Pressure on the brake pads will not be fully released and they will not retract from the disk, causing the disk, pads, and fluid to heat up the the point that the fluid boils resulting in the pedal going to the floor without apparent resistance as well as the smoke from the caliper.
The next morning the brakes will work fine (for a while) because everything has cooled and enough time has elapsed for the fluid to seep past the point of the obstruction back into the master cylinder.
I've never had it happen on my Porsche, but one of my pick up trucks had the same problem some years ago. I, too, was convinced it was probably a frozen caliper til an old hands-on mechanic who'd spent his life working on cars explained it to me.
Replaced the brake hose (actually replaced ALL the brake hoses) and the problem was remedied, never to return again.
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