Grindelwald used some curse or potion on Graves that put him into a state of suspended animation. That allowed Grindelwald to keep him alive but eliminated possibility that he will escape or that he will need to be fed etc. When Grindelwald is captured he refuses to reveal what he had done to Graves. Macusa still searches for him, but there could be anywhere Grindelwald could have stashed him, plus chance that Grindelwald killed him and disposed of the body is high. Eventually other concerns take up aurors attention and case of Graves dissappearance is shelved (perhaps after Picquery leaves office).

Cut to few years or even decades later. Maybe the curse starts wearing of, maybe some no-maj workers sumble upon Graves while clearin abandoned area, maybe Grindelwald, trying to clear his conscience after years in Nurmengard, finally confesses.

So Graves wakes up into a world which really forgot him and depending how long he was out, has to learn to live with it. Most people he used to know are now older than him!

Bonus:
- In intervening years, Graves has became something of a legend in the Macusa, especially among aurors