Well as of today and this history change, The Countess Dupre and Lt. Forbes went in alive and never came out.
Doctor Woodard also went in the secret room and discovered Barnabas' coffin there, although it was empty, he was pretty much on to him by then.
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During 1897 Gregory Trask finds Rachel Drummond there, I can't really remember why though, I think she was probably hiding out from him.[shckd]
1897-Sandor goes there on a mission sent by Magda to find some jewels and finds Barnabas instead.
1795-Angelique and Ben go to the secret room where Angelique plans on destroying Barnabas. Instead he rises up before she can stake him and kills her by strangulation.
Present Time-Jeff Clark meets Vicky & Julia in the Masoleum and Jeff suddenly has a flashback of being in the secret room and then opens it.
Although he definetely didn't go in there on his own, Jason Maguire was buried in there.