Stay within screaming distance. That is... So not comforting. He glances aside at John at the suggestion of whistling if they see anything and wonders just why exactly they're whistling if they're supposed to be within screaming distance of each other.

"Is there some particular reason we're whistling instead of. I dunno, speaking up to say hey, John. Can you come here for a second?" He looks between the pair of them before continuing. "We don't have to announce ourselves but whistling?"

He just has to ask. Maybe there's some secret Air Force strategy behind it all that Ianto doesn't know. Or maybe John is just making this needlessly complicated. A whistle will clue a listener-in to their location just as well as a call for help will, as far as he's concerned.