Technically it is not spoiler that Mr. Kaplan corpse was clearly shown, five seasons back, floating on the water after she suddenly decides to jump off of a bridge a moment later that Raymond has declined to pull the trigger (once again) to put an end to her life.
That was Season 4, Episode 22.
It is a blatant continuity error made by the writers to come up with such an imposing story line twist. That "corpse was never found" subterfuge makes no sense at all.
Raymond, for one, would never forget seeing the corpse of Mr. Kaplan floating on the waters of that river, not to mention the many FBI agents that were there, on that same bridge at the same time, when those events unfolded.
Even the most memory impaired watcher of the show would remember that scene. After all, Mr. Kaplan was a truly memorable character and a very much missed one.
Its One Bad Episode. Period.