这是省略句,补全后是All of them stared at him carefully,as though (they were) trying to understand something.
当as if/though引导的从句的主语与主句的主语相同,且从句中有be动词时,可以省略从句的主语和be 动词.又如：
He acts as if (he was )a fool.他做事像个傻子.
Tom raised his hand as if (he was going) to say something.汤姆举起手,好像要说什么.
She left the room as if (she was )angry.她离开房间,好像很生气.