All in all, he doesn't know if it's better or worse that they didn't close in behind him. But looking at them is a distraction from walking forward, so he does his best to put it out of mind.

After a few dozen yards of walking down the tunnel, it occurs to him that, though his mobile's signal might die down here... The Network might not be so restricted, especially in a place the notoriously echo-affiliated mice are congregating. Better to check before he gets too much further in.

He checks around the chalk walls for loose pieces to write with, settling for a pen in his pocket if there's none, and starts writing his sequence of numbers on some of the flintstone. If it works as normal... Maybe he'll let the others know what he's seeing.