Okay, I'll cop to it -- this is totally my fault. I get a little excited at the slightest opportunity to expound upon the setting, and in this case I added a small mausoleum to an otherwise boring graveyard where the PCs are investigating a rash of disappearances.
Naturally, they have now focused their attention on the mausoleum, to the exclusion of the rest of the graveyard, because, really, when people go missing in a graveyard, isn't the bad guy always set up in the mausoleum?
Unfortunately, this has left the players feeling stuck, since their exhaustive searches have turned up nothing relevant. I've tried to lure them out into the graveyard with sounds (e.g. cries for help) that I've suggested they should investigate, but they're terrified of wandering through the omni-present thick fog (my attempt at a little horror-esque atmosphere, maybe too good...). I've given them blatant clues that the mausoleum is a dead end, implying they should continue the investigation elsewhere, but still they hunker within it as if it were the sole thing keeping them alive.
I really don't want to break the "4th wall" and just tell them to move on, but if they don't move on the players are going to get really bored with the campaign that we've only just started. They've already done all the searching they can, so a simple secret door or something similar suddenly appearing within the mausoleum is not really credible at this point.
This is only my own particular case, but I'm sure other GMs have been in similar situations where their players get distracted by something irrelevant, so what are some general-purpose approaches that one could use in a situation like mine?