Since Rianna lacks Fix-It, there is not a whole lot she can do for investigating why the sensors have not been tripped in the tunnels.
Of your two theories, Zaltus finds it unlikely that he was just able to get out before tripping a sensor. They should have gone off the moment he went in.
"So he must have found a way to bypass them undetected," Zaltus says. "Not ALL of the maintenance tunnels have sensors installed, of course. But the major ones do."