We have 2013 vintage gardenia, and I have the house plans which specifically show a vent through the roof for the microwave. I have also notice cold air in the microwave in the winter on some occasions. I have no idea when or if the building codes changed, but I have heard storing of the cover plate on the microwave NOT being removed, which blocks the air flow the the vent. As for verifying if you have a vent, you should be able to see the rectangular vent from the attic access point, and you should be able to see the rectangular vent through the roof.
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Pennsylvania, for 60+ years, most recently, Allentown, now TV. 
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