The three surviving domain experts certainly couldn't have disappeared without a trace. Therefore, given the situation at that time, unless there was an accident, they were likely to have already retreated at that point.
After all, if those domain experts were nearby Zhou Tian, then when the other domain expert died and the destructive energy exploded, it would have been impossible for ordinary people to survive in such an environment. Hence, unless they were not nearby at all, even if they did not perish under that wave of destructive energy, they would likely have been forced out by it.
Since those domain experts didn't show up after the sweep of the destructive energy, even without evidence, Zhou Tian could more or less confirm the fact that they had already retreated. And upon discovering this, while Zhou Tian felt it was only natural, he was also somewhat unable to accept the outcome.