Combeferre can indeed picture it. He's seen it before. He's worn it before, he suspects.

"No, I don't think so," he says. "But please keep an eye on your watch? I'd like to try using the watches to communicate with you from the Labyrinth. I want to test it and see if it works."

Combeferre is still not finished playing with, and learning about, the watch's technology. Why miss an opportunity to do so?