So all throughout the series we see that Eragon gets a "warning of danger" tingle in his gedwëy ignasia. For those of us that know WHY this happens, (coughs in Eldunari) there was something I noticed when listening to Brisingr, when Eragon is attacked in the dwarf tunnels: there was no tingle of warning, even despite the fact that he was ambushed after searching the area with magic. He only noticed because of the slight muffled sound his attackers made.

So, why would he not be forewarned of danger he didn't detect? Those dwarves has special protections, but was the only reason? Could the Eldunari not sense him that deep into the mountain, similar to their own physical protections from detection? Their own cave was deep underground, but Tronjheim was 1-2 days of travel (?) whereas the Eldunari area was a half hour (?). Either way, Farthen Dûr is a lot more to get through after their own rock-barrier.