Detective Lyle is clearly mistaken about the "underground transport tunnels," which have windows, an obvious sign that this is an above-ground building entered at ceiling level. Also, low-lying New Orleans is known for its high water table; any hole in the ground quickly fills with water--hence the above-ground mausoleums like the ones at the cemetery Benji runs through. Water must be continuously pumped out of underground structures in New Orleans; decrepit abandoned tunnels would be perpetually flooded.
King's Pawn Shop shows an astonishing lack of security measures. Since large quantities of valuables and cash are kept on the premises, real U.S. pawn shops always have extensive physical security to prevent unauthorized entry, particularly at a concealed entry like the one Syd and Titus use (and for this exact reason).