Almost every family tree I have worked on has turned up family members for people who had no idea their kin were out there and sometimes very close at hand.
Several times in researching there has come the time when I have had to send out a generic type of letter to contact someone who may or may not be a relation to the tree I am working on at that time.
I have had many successes with contacting online researchers. They have made a point of updating their email addresses on the message boards and such and check their in boxes more than once a year.
Not everyone is on the internet however. Hard to believe I know. I find this to be the hardest part of the research game. I find a possible connection somewhere like an online phone book for the town I am looking at or an obituary perhaps and I create a letter describing who I am and what I am working on, keeping the confidentiality of the person on my end. More times than not this has to be translated into French, Italian or some other language which I don't speak.
I try to be as open as possible with the information I already have so as not to seem like I am identity phishing or something else underhanded, without betraying any confidences. I explain what it is I hope to find out and assure them that their own information will not be shared unless they would like it so.
Then you put this in the mail or email and you wait to see if there is anyone out there. You have so many scenarios on what will happen. Are they the correct family? Are they alive? Do they still live at that address? Do they care? Did they mean to answer you but accidentally deleted your email? Is it in their to do pile and not going to get done in their lifetime? Did the online translation you used actually insult them and their grandmother unintentionally?
So far I have had one letter returned, one answered but not the family I was looking for and several successes. For instance a first cousin in Germany for one man. They had no idea either one of them existed. Multiple cousins, aunts & uncles for a woman who didn't even know her mother had siblings. The mother had not spoken to her family for over 50 years but they eventually had a lovely reunion. Several others too but the biggest one by far was tracking down the half sister of my husband. She is much older than him and she didn't know about him. He knew he had half siblings but knew nothing about them. We have been emailing now for 3 years or so but have not met.
The latest one is 2 letters I had sent to Switzerland. They were done in Italian as they were going to one of the last Italian speaking cantons there. I took a chance. I found addresses for 2 of the 3 sisters I was looking for. Off they went and I waited. How long does it take to get to Switzerland and back if they are not delivered I wondered? A couple of weeks went by and I was starting to think nothing would come of these but surprise, surprise what came in the mail yesterday. A big envelope from Switzerland. Funny enough it was from the one sister that I could not locate, the other two having already passed away. She was so thrilled to be contacted and sent me a family tree that they have had done there. I don't have any sources other than her paperwork but it will be presented as such. I was able to confirm a couple of facts though. The best part is both parties involved are very eager to have more contact. That is good enough for me. Oh yes, I have found cousins for this person much closer to home as well. We will see if that goes as well when contact is made there. I am pretty sure it will.