[Miss Marple]

If an accomplice had the body and was holding out for more money or a favor, he might have moved it as revenge. Or perhaps the body was really there, and some of those stooges that were searching were paid not to find it.

[Helen]

Could be Condidit paid someone to get rid of the body in a way it would never be found (Condidit seemed sure it would not be). Whoever was to get rid of the body pocketed the $$, and figured he could deal with the body without going to the expense of a flight over the ocean or some such. After the search of Rock Creek Park, he buried it there. [There is no honor among crooks.] Just another theory.