Disclaimer: I am neither Whedon nor Bellisario. I am just playing in their sandboxes.
Crossover: BTVS / NCIS. Years after "Chosen", NCIS somewhere around "Hiatus".
Spoilers: Whole series for BTVS. References to NCIS episodes "The Meat Puzzle", with "Bete Noire", "Hiatus", and possibly "Reveille", "Twilight", and "Kill Ari", referenced in later chapters.
Chapter One: Hacking
Many people thought Quentin Travers had been the oldest Watcher alive... that is, before the Bringers and the First had gone after the Council and the Potentials.
Many people thought that most, if not all, of the old Watcher's Council had been killed off.
Many people were wrong.
While they were reconstructing the Council's financial holdings after the Sunnydale collapse, the new Council - now operating under the cover of the Giles & Rosenberg Detective Agency of Orange County - found one other living Watcher that they hadn't known about. It had taken a long time to find - the Council had so many accounts in so many financial institutions that it had taken years to find and gain access to all of them.
Mrs. Vanessa Mallard. All they had on her was a bank account number - the Council had been paying her a stipend by direct deposit, and since the bank itself had not detonated or collapsed into the mouth of Hell, Mrs. Vanessa Mallard was still getting a check every month. It was coming from an account Willow had not even found for years, at Navy Federal Credit Union out of San Diego; Mrs. Mallard's was at Bank of America, so no help there.
After a bit of digging, Willow hadn't come up with much information. Mrs. Mallard did not seem to have any accounts in her own name, other than the checking account - which itself was odd. No cell phone, no cable bill, no house, nothing. Her credit report looked like it had at one time or another been sanitized by a federal agency with a name derived from a random selection of three Scrabble tiles, and the date of birth listed sounded faked too, as it fell somewhere just before the time of World War I. No address was listed with either the credit bureaus or the bank, and the account had not been significantly withdrawn from in over twenty years; there was just enough activity to keep the account active, always in the form of checks written to a woman who turned out to be a Mary Kay supplier and mobile dog groomer in northern Virginia. The entrepreneur in question had, unfortunately, recently been murdered.
In fact, the most in-depth information Willow had been able to hack out of the 'Net was a single mention of a Mrs. Mallard - no first name - listed as being in protective custody for a two day period during the winter of 2004-2005. The report itself had referred to her as a 'family member of a Department of Defense employee' (left unnamed in the redacted report) whose safety was threatened as part of an 'ongoing NCIS investigation' (though there was no reference to a case file, probably also redacted). Willow had been unable to hack into the DOD's employee database to follow the trail that way.
The only useful information, actually, was the name of the author of the report, one Special Agent Anthony DiNozzo, whom after further checking it seemed was an agent with NCIS - the Naval Criminal Investigative Service, Willow found out - and was based at the old Navy Yard facility on the Anacostia River in eastern D.C.
A call to the NCIS office at the Navy Yard confirmed that DiNozzo still worked there, but was not currently in the office, and Willow left a cryptic voicemail and a phone number, hoping she might be able to use him to locate the elusive Mrs. Mallard. And that NCIS's Mrs. Mallard was her Vanessa Mallard, Watcher.