He wasn't aware there was more to say, though Peridot obviously disagrees. Childermass looks to her again, eyebrows raised ever so slightly as if to question her own question. The inevitable problem here is a different approach to being upset, which he isn't entirely unaware of. The gem had been upset even over Tyki going missing and she had hardly known that man, to begin with.

With Lambert, however—

"I do not know what else you would have me say," he admits outright. "I could ask what happened, but I already know how it ends, with the Prince taking him away."

Because he isn't here, so it must be that.