Sentinels on the walls/at the guard towers? They would probably notice an explosion of earth nearby... and deal with it promptly The dreamspike also doesn't stop at the walls.... that tunnel sounds like a deathtrap, if you ask me... if Taim's spot the entrance, they can follow you in. If you exit too early, they might spot the opening of the hole. Androl would have to keep digging until he finally reaches a point where he can again open a Skimming gateway (as he can hardly wait long enough to learn the ground).

Escaping is no doubt not impossible, but it would probably be very difficult and and long/involving, and would need some set up . It sounds more simple to have Androl find a way to open a gateway than go into a whole "escape from the BT" subplot, especially as the escape would merely be a prologue to the Light learning what is going on, and planning how to deal with it...

With RJ, when he planted something as obvious as this, it was usually, but not always, a good sign it was a red herring. Brandon isn't exactly as subtle. Androl's talent is a loaded gun left on the mantle in act 2.. chances are great Brandon intends to shoot something with it in act 3, IMO.