Adrian and Frey did not stop with the obvious area that has been claimed. They do not have the time to worry about others. Even if they could enter the cave to verify it is empty, the path to the fourth layer was close to them. They only needed to make sure that they enter the fourth layer without any problems.
Adrian could also see some traces of a difficult battle from the safe zone before them. This means that whoever defeated the monster here must have been recuperating. There were also traces of lightning strikes with burnt ground which meant that they also got struck by it. Since the other party was obviously inside the cave and did not even attack them when they landed, Adrian decided that it was not worth to fight them now.
