Dead people are harder to find. With all the info Kenji had, it was a piece of cake to surreptitiously run River's credit card charges. In the old days, it wasn't so easy.
Once Judy Garland needed to find her then husband to tell him something, but didn't know where he was. She called FBI Director, J. Edgar Hoover. Hoover called back 30 minutes later and told her the city he was in, his hotel and room number, and the assumed name he was registered under. All before computers.