Well, we just moved into this house not so long ago and live on Rt. 231 and haven't had the time or the money to build a fence yet - so in the middle of the night when he has to go outside we sometimes just chain him up - or when we're to be gone for an extended period of time. He's mostly an indoor dog, though. I guess we just figured a chain was better than letting him run free by the highway . . .
Please post very big fliers everywhere!! Deliver them to your neighbors. Post in the grocery stores, etc in Calvert and McKay's, Food Lion, etc in St. Mary's. Not sure which side of the bridge you're on. Call and report it to the Tri-County Shelter and Humane Society.
On a side note - if he was a barker at night, someone may have let him off the chain unless you can tell it was broken. Also, there are many wild animals that can harm a dog, especially if it is tied. Don't know how old he is but he should be able to go through the night without going out. Maybe feed/give him water earlier in the evening and let him out right before bed.
An alternative to a complete fence could be an enclosure until a fence can be built. You may be able to find some pretty inexpensive ones in the ads. Something like this with a lock on the latch -
dog kennel/cage for out side
Good Luck and I hope he makes it home quickly.