Following is a summary of this Breitbart article:
Several Arab and Western intelligence agencies have reported that tens of thousands of ISIS fighters have “gone missing” in Syria, Iraq, and Libya; not being found where they are expected to be at the conclusions of conflicts. Battles “did not yield thousands of dead or captured fighters” as expected, according to an Arab intelligence official.
“That’s why it’s worrisome – these people are gone. They either integrated into noncombatant communities or, more likely, infiltrated into neighboring countries – to Turkey, and from there to Europe and other Middle Eastern countries, and in Libya’s case, all over North Africa,” the official said, also mentioning that 700 Tunisian fighters are unaccounted for, while thousands more are being sought.