Actually, in MD, on certain roads (I-95 in particular) having certain out-of-state license plates has been used as RAS for stops to search for "illegal firearms". Plates from VA, NC, and TN are particularly "suspicious"...
As for the Carry Permit info atttached to your DL, there are a LOT of people who will tell you there is no cross-state database that local police or State Troopers have access to. This is false.
Almost ALL the States have access to several hemisphere-wide criminal databases, which are comprised of links to all the other state's info. These database systems also include Canada and Mexico, and have been operational for nearly a decade. By 2015, the plan is to have these systems FULLY operational, so that any LEO in ANY place in this Hemisphere will have access to driving records, vehicle registration, and criminal records databases of ALL participating parties.
Participating parties include all the provinces of Canada, all the states in the USA, and all the states in Mexico.
So if you have a CC permit, that info probably WON'T come up automatically when an out-of-state LEO runs your DL, but your DL will most likely be flaggged as having "additional criminal records information" and the LEO will then have to click through one or more additional pages to get to your CC permit info.
But do not be deceived by those who don't understand how electronic information sharing works today in the US--the info is there, and is available to about 75% of ALL the LEOs in the USA, and to nearly 100% of LEOs in urban areas.