Keep watching the coolant level. It doesn't take much (a few drops) to give you that puff of smoke, so I would not rule out yet a head gasket or cracked block. But if it is a very slow leak, you can probably keep on top of it for a while. It might only show up on an overnight pressure test started when the engine was hot, so it could be quite difficult to prove.
Could it be gray smoke? That could be a little bit of oil, which might slip past the rings while the car sits after shutting it down. Not much you can do about it, just check the oil and coolant frequently and hope for the best.
Did your mechanic pull the spark plugs? If you are burning oil or coolant in one cylinder it will leave residue on the plugs. If they've never been changed, that would be good maintenance at this point, too.
It could also be simple water vapor, which is a normal combustion byproduct. The water condenses in the exhaust system and boils off in the morning. It's less noticeable when the engine is hot so you only see it on start. But Florida is pretty warm already so I don't think that is likely, I just throw it out as a possibility.