Does it matter if the car is on your private property. Are these rules only for public streets, etc?
If there is frost on my windshield I usually leave my car running for a few minutes.
I don't believe it is a violation of Maryland law if we're talking about private-use private property (i.e. property that is not only privately owned, as with a a shopping center or such, but is also not used by the public in general). At least, if someone has definitive knowledge to the contrary, I'd appreciate a pointer to the statutory provision under which it would be a violation. (To be clear, I'm not referring to provisions of more general applicability, e.g., with regard to noise violations.)
From the past (search "car starters" for example), the answer you are looking for is "YES" it is illegal to have your car running unless you are either with your car (I believe inside it or outside it, not 100% sure on that) or you have a car started installed that will disable the vehicle unless the key or transmitter on some cars is in use before putting the vehicle in gear. If you put the vehicle in gear without one of these it should shut the engine off.
Can you refer me to the relevant statutory provision? The title scope provision of the Rules of the Road Title of Maryland's Transportation Article would seem to mean that, e.g., § 21-1101 (which is part of the Rules of the Road Title) wouldn't apply to vehicles on non-public-use private property (such as, e.g., in my driveway).