Do you mean you SEE a marker on your map ?
That's because of the CURRENT Main Quest (correct me if I'm wrong)
(main quest : Dragon No More)
Have you solved the Quest "Lovis' Loot ?
(quest given by Romon the Priest in Broken Valley Village)
If yes: there's nothing to do anymore,
besides trading with Carlin (if you have not killed him ).
But sorry, I know nothing about a hidden chest nearby the Blood altar of Amdusias .. if so, let me know
