With the artefacts collected, a subtle mental pressure appeared in the group's mind. No one said it aloud, but they all felt it. They were worried that they would be attacked and would lose it. Or more berserkers would come. Or something else unexpected would go wrong.
They tried to focus on their surroundings, but after all the hassle they went through to get the artefacts, it made sense that they were worried about it.
Even Nero felt that subtle urge to just rush back to the ground floor and give in the artefacts as quickly as possible. But feeling it did not mean that is what he did.
There were still many corridors on the third level they had not personally explored, which meant that there was a chance more artefacts could be hidden there. Nero had no intention to steal other things, but it was for this reason specifically that he also went through Arter's and everyone else's backpack. He did not mean to invade their privacy, but what if they had artefacts?