Depends upon exact phrasing to me; and even then, I have questions at times. As you know, every ad is written by the person who is looking for someone and they all have their ways of phrasing and capitalization.
"OF MARY BRENNAN, wife of John Brennan. They left London, England in 1868 and sailed to America. When last heard of, Mary was living in New York City. Address to her sister, Bridget Maguire, in care of John Hynes, 1018 North Tenth St., New York."
You can pick hairs here but this is what I would interpret:
"Mary" as given
"Brennan" as surname
Skipping maiden because we don't know that for sure
"Self" as relations
"London, England" as departure place
"1868" as departure year
"America" as arrival place
"New York City" as last known location
same as line one changing the given to "John"
"Husband" as relations
same as line one for departure place and year
rest is blank because John's name is not mentioned as living in New York.
"Bridget" as given
"Maguire" as surname
"Sister" as relations
rest is left blank
Now if you change to:
"OF MARY BRENNAN, wife of JOHN BRENNAN. They left London, England in 1868 and sailed to America. When last heard of, Mary was living in New York City. Address to her sister, Bridget Maguire, in care of John Hynes, 1018 North Tenth St., St. Louis, Mo."
In both ads, John might not be missing at all because he is not mentioned as living in New York with Mary. John could be very well be interpreted as missing in the second ad because his name is capitalized. From what I am reading on this post and others, the relations part of John's line is up for debate. One person might key Mary as line 1 the same, key Bridget as line 2 since she is Mary's sister and you would have to put her under Mary to show that, and key John as line 3 and "Self" as relations thereby dropping the fact that he is Mary's husband because his name is capitalized and interpreted as missing.