When he first heard the sound, Locke did think of rocks and he looked around as if expecting the cave they'd just come from to show signs of collapsing. There didn't seem to be anything like that and his attention was yanked away by the arrival of a skeleton on a boat coming up to them.

Locke took a step back from the shore again, distrust clear on his face. Where he came from, the only skeleton things were monsters and he had no reason to think this thing was any different; Well, except for the fact that it was talking.

None of the monsters had ever talked except Atma Weapon. More than anything Locke wanted to go for his daggers, but they didn't have any weapons between them and he was left with just his feeling of caution and distrust. After all, if it was part of the institute, there was no good to be had from it.