In Singapore they were already nabbed. Thailand is not the kind of place a bunch of Arabs could easily purchase explosives surreptitiously. In Vietnam they'd be lucky to cross the street without generating a police report. In Cambodia they'd be instantly robbed and killed in the course of buying their armaments (Hun Sen would not welcome their presence, and even the KR remnants wouldn't willingly sell explosives to Arabs in Cambodia). In Taiwan they also would be more than a little noticable.
These guys are completely delusional to think their activities would go unnoticed in SE Asian countries. They'd stand out like sore thumbs, and there's little chance of an Arab living "anonymously" in SE Asia (the crackpot notion that they could do so in Vietnam is particularly hilarious).
Indonesia is the one remaining problem, since there is no law there whatsoever, and they practice the Religion Of Peace (tm).