Vanunu has no secrets to divulge. What Israel is fearful of is Vanunu drawing the attention of the world to its huge illicit stock of nuclear warheads. This wouldn't help when it is trying to persuade the world to whack Iran for merely ATTEMPTING to acquire a nuclear weapon. So it wants Vanunu muzzled, even though he has long served his sentence, which was a savagely vindictive one by any standards.
In three days, Greek coastguard rescues 2,500 migrants (Reuters)
from the article: Vanunu to return to prison for violating the terms of his parole