I travel to western Pennsylvania from Virginia often. I used to travel through Maryland to get there, but my current route totally avoids Maryland by driving through the hills of West Virginia. Depending on which part of PA you're visiting, you might consider using WV instead of MD to get back to VA.
There's nothing better than reading the Maryland law itself to learn what the law is. Hearsay usually either gets you into trouble or hinders your liberty perhaps more than the laws themselves could. Here's my non-lawyer synopsis, followed by links to the specific statutes online. It's not the best web-site, but here's the main Maryland Statutes web site.
The firearm just has to be "unloaded" and in an enclosed case or enclosed holster. I am not a lawyer, but as I understand the code you can have a full magazine anywhere you want. It is unclear in the code, but perhaps defined in case law, whether "unloaded" refers to the chamber; or if it refers to not having any ammunition attached to the firearm in any way, such as a full magazine inserted into the firearm. I don't see anything written in law that prohibits the carrying of a full magazine that is not installed in the firearm; with that you should be fine.
It seems pretty clear that you may carry the firearm in a holster, even on your person, so long as it is unloaded and the holster completely covers the firearm; and that you are going to/from your residence, business, property, target shoot, etc... So locking everything up in your tool box should be more than sufficient. There is no duty to inform an LEO either. Just keep all things related to firearms out of sight and there shouldn't be any reason for an LEO to act on section 4-206.
I think there are three main laws that you would be concerned with when traversing Maryland. They are in the Maryland Criminal Law Article; sections 4-203, 4-206, and 4-209. Additionally, Maryland has banned specific weapons they deem "assault pistols" with pretty steep penalties, all laid out in 4-301 thru 4-306.
Maryland Criminal Law - §4–203.
Maryland Criminal Law - §4–206.
Maryland Criminal Law - §4–209.
Maryland Criminal Law - §4–301.
Maryland Criminal Law - §4–302.
Maryland Criminal Law - §4–303.
Maryland Criminal Law - §4–304.
Maryland Criminal Law - §4–305.
Maryland Criminal Law - §4–306.