My mother's closest autosomal match that is not a known relative is an estimated 3rd-5th cousin. Of all my mother's matches, this person shares none in common, no significant segments with anyone else, nothing. Just a big old chunk of chromosome 16 (18cM) and 8 other little bits of segments 3cM and less. They also haven't responded to my email in almost a year and are not on gedmatch, no family tree posted online... nothing. I've taken this to mean one of two things...

1)It's just a rare family group and by chance no one else has tested
2)By chance both my mother and the match happened to inherit a "strong" or "sticky" segment from a certain set of ancestors and by chance, other "matches" are too distant to be picked up by the matching algorithms.

I've researched both my mothers and this match's family trees back anywhere from 5 to 7 generations and while my mother has some fairly specific ancestry, the match is a total mix up and some countries of origin just don't make sense with my mom's, so that helps narrow down the search... but still no common ancestors or surnames from common places jump out. Also no X-match. And no match to the other known relatives of mine who have tested, who are all from my mother's paternal line.

Should I guess that maybe the match is further back then the 3rd-5th generational distance estimated?