Castiel didn't react to the webbing, but it did surprise him somewhat. He looked down at his plate, at the webbing right next to his food. It looked like spider's silk.
He reached down a hand carefully and touched it, flicking it at first, then touching it more completely when he felt no poison or ill effect.
He pulled it off his plate with some difficulty, and held it up to the light, studying it for a moment.
"But you're not a demon," he observed, using his napkin to pull it off his fingers. The residue didn't bother him overly, and he slowly nodded, smiling a bit more and putting it down.
Then he felt a strange bubble in his throat, almost like he was going to be sick. Before he could stop it, it came out, in the form of a low, breathy laugh.
He leaned back in his seat as the laugh grew. By human standards it was very low and soft, but for an angel he might as well be roaring with laughter. Of course, if he truly did that it was likely everybody in this section of the City would die.
After a moment he quieted, shaking his head. "My apologies," he said, still smiling. "I had started to think I'd been shown everything. I should have known better." A man who embodied one of most humans' worst fears, wasn't a demon, and spent his days charming waitresses and feeding the homeless. No, he hadn't considered that one.