I would think the Last Will and Testament would include people that she was leaving her estate to and may include their addresses or the towns where they lived at the time. The list of interested persons might mention family members as well.
Thanks Shelley 
I've avoided naming names of living persons here, so you'll have to fill in the blanks!
I thought I would have a look at the Maryland probate record for Phyllis McGuire. In case you haven't seen it, the personal representative listed on the search results looks to be a close relative, living in 2007 in Gaithersburg. Since no attorney is listed on the probate record, that person most likely administered the estate. And, if you wrote to the Gaithersburg address that is on the probate record and received no reply, then it is is likely that the p. representative moved between 2007 and when you wrote the letter. A person with the same name is listed on beenverified.com in Montgomery Village, MD, which is a neighboring town.
I also noticed the mention of Matthew's family going in 1951 to Cedar Grove, New Jersey in the thread. That prompted me to search for him in the U.S. census records on familysearch.org. There are 1920 and 1930 U.S. census listings for Matthew McGuire, 8 and 18 years old, living with his family at 437 Broadway, Elizabeth, New Jersey, which is 18 miles south of Cedar Grove. In the 1940 U.S. census, the family is still at the same address in Elizabeth, but Matthew McGuire is not with them. He is in the U.S. Army Air Corps at Mitchel Field in Hempstead, New York (on Long Island). Matthew seems to have enlisted in the U.S. Army as early as 1935, since he is listed on the 1940 census page as living at the same place in 1935. Here's a link to the 1940 census page:
https://familysearch.org/ark:/61903/3:1:3QS7-89MB-SZ72?mode=g&i=27&cc=2000219. Matthew must have served in the Army and then re-enlisted in or transferred to the U.S. Navy.
There seem to be a boatload of McGuires currently living in Cedar Grove, New Jersey. Perhaps Matthew and family were going to visit relatives there. Perhaps relatives of his still live there.
Hope this helps.
John
