Some states have a "Declare" requirement (Michigan???) if you have a CPL/CCW/LTC, whether you are actively carrying or not.
I've asked an ex-JBT friend of mine how this scenario would play in Massachusetts, his advice was "No cop on the side of the road is going to care about a plastic card in your wallet, but you damned well better declare if you are actively carrying... And turn on the interior light, and keep your seatbelt on, and lace your hands through the steering wheel, and the first thing out of your mouth better be 'Officer, I have a License to Carry and I AM armed at this time'"
I guess in Maryland the "No cop on the side of the road" part of his analysis doesn't really apply.