The heroes find a small crypt somewhere and being brave adventurers they want to check it for evil forces (=graverobbing). Or for any other reason .
The crypt is undisturbed and with some dangerous traps it
includes a bit treasure . The coffin too. BUT , there is no corpse found anywhere in that crypt. It is also certain that it wasnt opened or disturbed since building. No secret doors/things either. No ghosts or undead for that matter.
Should the players make no conclusions about this , point to the fact that the usual religious ceremonies have been carried out (whatever it means and however it is shown)-
which is not done if the body is missing.
If you wish to force them to some further investigations,
make them subjects to some kind of curse,better not serious but annoying.
OR better yet , let them decipher the name of the ‘user’ of the crypt, accidentally being the same as one of the heroes’ family name.
I dont know any solutions to this , but it may be interesting to adapt the players theories and twist them someway.