I don't know if anyone of these are the one you are looking for but I thought I would throw them out there for you.
Go Be A Hero (http://www.fanfiction.net/s/9120215/1/Go-Be-A-Hero) by TheWrambler Nineteen year old John Watson is ready to leave for Afghanistan. But he is not ready for what he is leaving behind. Teenlock.
Missed Calls and Suprising Returns (http://www.fanfiction.net/s/8141359/1/Missed-calls-and-surprising-returns) by Elfprinzess Lestrade and the other policemen/women didn't believe Sherlock had a friend. Too bad he actually has a boyfriend. NOT cannon, John already knew Sherlock before leaving to fight in Afghanistan.
Homecoming (http://www.fanfiction.net/s/7762034/1/Homecoming) by PeaceRoseG'ladheon Sherlock always has, and always will be alone. Or so the Scotland Yard thought, until a soldier, just back from Afghanistan, appears at one of the crime scenes, looking for a Sherlock Holmes. For detectives, they're terrible at making accurate deductions.
A War Every Week (http://www.fanfiction.net/s/9375812/1/A-War-Every-Week) by SherlockedSherlockian Sherlock's 23. John's 25. Sherlock's in London. John's in Afghanistan. While Sherlock Holmes, consulting-detective-to-be, completes University, John's away in a foreign country on military service. With two childhood friends separated by miles, one in love and the other falling fast, will there ever be a happy ending? Co-authored with the brilliant CowMow as John Watson. Johnlock.
I know of another one I can't find, but I hope you enjoy these. Rairakku
Some Possiblities....
Go Be A Hero (http://www.fanfiction.net/s/9120215/1/Go-Be-A-Hero) by TheWrambler Nineteen year old John Watson is ready to leave for Afghanistan. But he is not ready for what he is leaving behind. Teenlock.
Missed Calls and Suprising Returns (http://www.fanfiction.net/s/8141359/1/Missed-calls-and-surprising-returns) by Elfprinzess Lestrade and the other policemen/women didn't believe Sherlock had a friend. Too bad he actually has a boyfriend. NOT cannon, John already knew Sherlock before leaving to fight in Afghanistan.
Homecoming (http://www.fanfiction.net/s/7762034/1/Homecoming) by PeaceRoseG'ladheon Sherlock always has, and always will be alone. Or so the Scotland Yard thought, until a soldier, just back from Afghanistan, appears at one of the crime scenes, looking for a Sherlock Holmes. For detectives, they're terrible at making accurate deductions.
A War Every Week (http://www.fanfiction.net/s/9375812/1/A-War-Every-Week) by SherlockedSherlockian Sherlock's 23. John's 25. Sherlock's in London. John's in Afghanistan. While Sherlock Holmes, consulting-detective-to-be, completes University, John's away in a foreign country on military service. With two childhood friends separated by miles, one in love and the other falling fast, will there ever be a happy ending? Co-authored with the brilliant CowMow as John Watson. Johnlock.
I know of another one I can't find, but I hope you enjoy these.
Rairakku