If Darien can hear Bach coming before he even sees him, in the flying belt, then why can't the compound guard hear Darien coming ahead of time? It has been established that Darien's senses are advanced beyond those of present day humans.
During the flying scenes, the image sometimes is reversed as evidenced by the palm controller switching sides.
Raoul and Jane may both be gone, but Raoul's associates are still alive and know where Dr. Bach lives -- he can still be kidnapped again and forced to create another flying belt. Surely any terrorist government would be willing to invest the money necessary to get such a device and constantly be after Dr. Bach.
When Darien used the belt to blow up the gate, the unconscious guard behind Bach looked up to watch the explosion.