John shuffles along on the narrow bed, making space if Ianto wants to sit down and see better.

"Yeah, I did that once. I can --" and he gestures at one of the dots vaguely, which changes to a different blue. "Tag you a different colour so I can find you if we get split up. It's pretty cool, can pick up energy readings too. Like radiation, power sources. That kind of thing."

He squints thoughtfully, then looks up and frowns at the wall opposite.

"I'm not exactly sure what they use for power here, come to think of it."