Another Possibility

Before exploring the area, they tried to gather the remaining demon crystals they could get from the demons. Most of the demons that Noel killed didn't have anything left, so they searched for the demons they killed. 

They were already quite lucky that they got about three thousand demon crystals with variety grades. 

The most important ones would be the twenty peak level crystals and more than a hundred advanced level crystals. There was no Superior Demon this time though. The Supreme Devil Organization might be planning to use it during the actual siege.

After loading everything in the carriage, they rebuilt their camp on the other side of the river to avoid the pungent blood smell. 

…

The next day. 

Noel and the others continued their journey to the base of the Supreme Devil Organization, only to find it nowhere to be found. That was right.

The base had been destroyed completely with the only clue being the people that fled earlier.