I actually have a third idea. How about people head off with the intention of coming back in the morning but during the night or in the morning receive a telephone call or telegram warning them not to return to she mansion but instead report to the S.S Demeter in south Hampton. The idea is that the mansion is under surveillance and is no longer safe.

This will allow us to move the action forward and loop in the antagonists. One of the more action oriented characters could even be the one to spot it and put out the warning