The case of the missing anti-freeze

[Dave Rifkin]

Okay guys I have a situation that has puzzled my mechanic for the past few weeks so I am reaching out to the experts for suggestions.
I have my '69 Corvette at a local shop for some work and they called me with a concern about the anti-freeze level in the car.
Apparently they started the car up and noticed a puff of smoke coming from the left tail pipe. The mechanic thought he could smell anti-freeze so he checked the radiator and noticed that it was half empty.
He pulled the plugs thinking that he'd find one, or more, with a "steam cleaned" apperance if there was a coolant leak but, no luck.
Today he called me to say that he pressurized the system for about 30-40 minutes and the car only lost maybe 2 lbs over that period of time.
I had them change the oil and no mention was made of coolant in the oil.
The car starts up fine and runs great but, there is this puzzling issue of the missing coolant.

Do you guys have any ideas as to what could be the problem? At this point the mechanic is leaning towards a problem with the heads (porous head or a cracked head)

Thanks,

Dave